Brake Softness after Prolonged Idle at Red Light - Update

[BrewCityCobra]

I'm facing a strange brake situation and wanted to see if anybody had experience the same problem before or had any ideas on a fix.

For background, I have the FFR pedal box, with dual Wilwood Master cylinders (as provided) and full Wilwood brake kit. Brake lines are stainless (pictures can be found here to the extent it helps: https://thefactoryfiveforum.com/showthread.php?38979-The-Brew-City-Cobra-427W-IRS-Rear-Wheel-Well-Vinyl&p=454756&viewfull=1#post454756). I'm running Wilwood 600 Plus fluid.

So the last time I took the car out, the brakes worked fine - that is - until I was stuck at a red light idling for a prolonged period. After a few minutes at the red light I noticed that my brake pedal was slowly going soft (e.g., slowly depressing toward the firewall despite me providing a constant pressure on the pedal itself). This slow movement continued for a bit before I took my foot off the brake and re-applied, at which time the brake re-engaged where it typically did and felt firm. After a little bit longer the slow softening returned - and I re-applied. This happened two or three times before the light turned green.

After driving a down the road a bit after the incident when I did need to re-apply the brake everything felt fine.

When I got back to the garage I checked the reservoirs and everything seemed fine (none were particuarlly low). I tried to re-create the incident (e.g., sitting there with the car off, but with my foot on the pedal) but it held firm.

So my question to the group - what the hell is going on?

I have two theories:

1) The portion of the brake line that is closest to the header is getting too hot when I'm sitting at idle (and there is no airflow in the engine compartment) causing the fluid to boil temporarily. Obvious fix here is to just add a small heat shield to the header / brake line and hope that does the trick.

219705

2) One of my two master cylinders is starting to go.

3) I have a leak somewhere - but I don't see anything on the garage floor.

Any thoughts?

[lewma]

My uneducated guess would be brake fluid getting too hot ? If you've checked the reservoirs and they're at the same level before and after the drive, there's no leak.

Are you using the correct DOT brake fluid ?

those are my guesses
mark

[gbranham]

Did you check the driver's floorboard for wetness? There's been some Wilwood MC failures, and I'm loathe to resurrect the subject, but there have been some failures. Can you remove the top access cover on the driver's footbox and feel around the MCs for leaks? If the MCs are leaking, you won't likely see it on the garage floor.

FWIW, my brakes line looks similar to yours (the look at the 'T'), and the run from the footbox to the 'T' is even closer to the headers, and I haven't experienced this. I also have a 427 SBF.

Greg
219708

[rich grsc]

No way header heat would affect the brakes.I guarantee you could hold the brake line at the header

[JohnK]

If I had to guess, my bet would be that your brake fluid needs flushing. The thing with most high-performance brake fluids is that they're great when new, but they are much more hygroscopic that regular DOT4 brake fluid, so they need to be flushed much more frequently. A good indicator of how hygroscopic a brake fluid is, is to compare the dry and wet boiling points of the fluid. Fluids like Wilwood 600 Plus, Endless 650, Motul RBF 660/700 are great when fresh but if they're not flushed at least every 12 months they degrade significantly. Personally I really like Castrol SRF. It has a dry boiling point nearly as high as the others (I've never felt any degradation even during hard track use) and a much higher wet boiling point, so you can get away with slightly longer flush intervals (but still not as long as the 2 year flush intervals for street fluids.)

If there are no signs of leaks anywhere, I'd start by flushing the fluid. Putting a small section of heat shielding around that section of line can't hurt, but honestly there's no way that fresh racing fluid should be boiling, even if the line is close to the header.

Here's a quick ChatGPT comparison, for reference:



Fluid
Dry Boiling Point
Wet Boiling Point
Notes / Comments


Wilwood EXP 600 PLUS
626 °F (~ 330 °C)
417 °F (~ 214 °C)
High-end racing fluid with strong dry spec


Endless RF-650
≥ 622 °F (~ 328 °C)
≥ 424 °F (~ 218 °C)
Very competitive racing fluid, better wet spec than many


Castrol SRF
~ 590 °F (~ 310 °C)
~ 518 °F (~ 270 °C)
Strong wet spec — resists degradation with moisture


Typical DOT 4 Street
~ 446 °F (230 °C) (minimum)
~ 311 °F (155 °C) (minimum)
Baseline for “everyday” glycol-ether / borate ester fluids

[cv2065]

Seized caliper pistons? If it's excessively dragging it could be heating up the brake fluid causing the spongy feeling.

[Caddy Dad]

I'm facing a strange brake situation and wanted to see if anybody had experience the same problem before or had any ideas on a fix.

For background, I have the FFR pedal box, with dual Wilwood Master cylinders (as provided) and full Wilwood brake kit. Brake lines are stainless (pictures can be found here to the extent it helps: https://thefactoryfiveforum.com/showthread.php?38979-The-Brew-City-Cobra-427W-IRS-Rear-Wheel-Well-Vinyl&p=454756&viewfull=1#post454756). I'm running Wilwood 600 Plus fluid.

So the last time I took the car out, the brakes worked fine - that is - until I was stuck at a red light idling for a prolonged period. After a few minutes at the red light I noticed that my brake pedal was slowly going soft (e.g., slowly depressing toward the firewall despite me providing a constant pressure on the pedal itself). This slow movement continued for a bit before I took my foot off the brake and re-applied, at which time the brake re-engaged where it typically did and felt firm. After a little bit longer the slow softening returned - and I re-applied. This happened two or three times before the light turned green.

After driving a down the road a bit after the incident when I did need to re-apply the brake everything felt fine.

When I got back to the garage I checked the reservoirs and everything seemed fine (none were particuarlly low). I tried to re-create the incident (e.g., sitting there with the car off, but with my foot on the pedal) but it held firm.

So my question to the group - what the hell is going on?

I have two theories:

1) The portion of the brake line that is closest to the header is getting too hot when I'm sitting at idle (and there is no airflow in the engine compartment) causing the fluid to boil temporarily. Obvious fix here is to just add a small heat shield to the header / brake line and hope that does the trick.

219705

2) One of my two master cylinders is starting to go.

3) I have a leak somewhere - but I don't see anything on the garage floor.

Any thoughts?

I had the same thing happen to me! I also have the Wilwood master cylinder setup. The master cylinder for the front brakes started leaking into the boot around the piston rod. I peeled back the boot and fluid poured out. Fortunately it was the most inboard master cylinder so it wasn't too hard to replace. I called Wilwood and they replaced it no charge. Hope this helps!

[BornWestUSA]

Is the fluid level in the reservoir going down? Can you find any leakage anywhere? (as stated above, check under master cylinder boots)

You are describing a master cylinder failure IMHO. Holding steady pressure with a slowly sinking pedal=bad master cyl.

[Jeff Kleiner]

Bad master slowly leaking under pressure.

Jeff

[Mike.Bray]

Surely it couldn't be a Wilwood MC failure! What would be the odds of that?

[J R Jones]

Brake fluid is less likely to boil under pressure. Same physics apply to a pressure cooker, pressure raises the boiling point. You could have a master cylinder piston by-pass leak, a small one. Porous casting?
You did not mention a brake booster. If you have a vacuum booster it could be leaking.
jim

[BrewCityCobra]

Did you check the driver's floorboard for wetness? There's been some Wilwood MC failures, and I'm loathe to resurrect the subject, but there have been some failures. Can you remove the top access cover on the driver's footbox and feel around the MCs for leaks? If the MCs are leaking, you won't likely see it on the garage floor.

FWIW, my brakes line looks similar to yours (the look at the 'T'), and the run from the footbox to the 'T' is even closer to the headers, and I haven't experienced this. I also have a 427 SBF.

Greg
219708

Unfortunately that thread and Wilwood's questionable track record of late in the master cylinder department is exactly where my thoughts went as soon this happened. Seeing your layout all but confirms that "heat" isn't a likely problem - especially if you haven't experience a similiar issue. Looks like I'm going to be checking some master cylinders tonight.

[BrewCityCobra]

My uneducated guess would be brake fluid getting too hot ? If you've checked the reservoirs and they're at the same level before and after the drive, there's no leak.

Are you using the correct DOT brake fluid ?

those are my guesses
mark

I'm using Wilwood's racing fluid - should be good to 620 degrees. Best I could find.

I did check the reservoirs and none seemed low, but I can't confirm there wasn't a bit of drop since it's been a while since I've look at them.

[BrewCityCobra]

I had the same thing happen to me! I also have the Wilwood master cylinder setup. The master cylinder for the front brakes started leaking into the boot around the piston rod. I peeled back the boot and fluid poured out. Fortunately it was the most inboard master cylinder so it wasn't too hard to replace. I called Wilwood and they replaced it no charge. Hope this helps!

Well that seems to confirm what the rest of the comments here are saying as well. I'll check the boots today and see how things look. Appreciate the response!

[BrewCityCobra]

Brake fluid is less likely to boil under pressure. Same physics apply to a pressure cooker, pressure raises the boiling point. You could have a master cylinder piston by-pass leak, a small one. Porous casting?
You did not mention a brake booster. If you have a vacuum booster it could be leaking.
jim

No brake booster on my build.

[BrewCityCobra]

Surely it couldn't be a Wilwood MC failure! What would be the odds of that?

I know, right.

Looks like the writing is on the wall here. I was really hoping that wasn't going to be the case. I'll be checking that this weekend.

Assuming that is the case, it really is unbelievable how many failures Wilwood has had just within the community here. Considering it should be a premium part and a brake part to boot.

[Mike.Bray]

it really is unbelievable how many failures Wilwood has had just within the community here. Considering it should be a premium part and a brake part to boot.

It's not a new issue, this goes back at least 30 years that I know of. Really pretty scary.

[cv2065]

I know, right.

Looks like the writing is on the wall here. I was really hoping that wasn't going to be the case. I'll be checking that this weekend.

Assuming that is the case, it really is unbelievable how many failures Wilwood has had just within the community here. Considering it should be a premium part and a brake part to boot.

Hopefully you don't have to replace one, although the brake MCs don't look 'that' difficult to replace. I've got a Tilton for the clutch MC. Let's hope it never goes down. It will be rough getting that out of there as they are larger and don't have a flat spot on the rod to rotate it out of the clevis like the Wilwoods do. I'm actually not sure how it would come out.

[Mike.Bray]

I've got a Tilton for the clutch MC. Let's hope it never goes down. It will be rough getting that out of there as they are larger and don't have a flat spot on the rod to rotate it out of the clevis like the Wilwoods do. I'm actually not sure how it would come out.

The 76 series are a smaller and use banjo fittings for the inlet and outlet. Easier to get in and out.

219809

[Jeff Kleiner]

The 76 series are a smaller and use banjo fittings for the inlet and outlet. Easier to get in and out.

219809

X2. And as I’ve reported elsewhere the 76 WILL work for a Mk5.

Jeff

[cv2065]

The 76 series are a smaller and use banjo fittings for the inlet and outlet. Easier to get in and out.

219809

Fittings aren't the issue, as I'm using the stock fitting that I had plumbed for the Wilwood clutch MC and that's pretty simple. It's more getting the threaded rod out of the clevis as even with a little anti seize, those rods are hard to turn by hand and if you can't get a wrench on it, you'd have to use pliers are something else to get in there. Not impossible, but those installing Tiltons should put a flat spot on that rod.

[BrewCityCobra]

UPDATE: So a quick update on the situation. Taking the board's advice I checked both brake master cylinders by pulling back the rubber boots. Both are dry. As an experiement, I also just sat in the driver seat, put my foot on the brake with standard pressure (with the engine off) and sat there for a minute or two. No "softening" occured. Finally. I looked into each reservoir and none appeared low (but I don't really have a reference here of where they all started, so hard to tell).

Is there another test I should be running to check the master cylinders?

My next test (unless the board has another idea) is to put the car up on the lift and spin each tire, see if any of the brakes are dragging.

[edwardb]

Everything you're describing is consistent with either air in the system or a failing MC. Absence any other leaks, e.g. at the calipers, brake lines, etc. Based on the intermittent failure, likely it's not air in the system. There are two seals on the Wilwood MC piston. If the one nearest the pushrod leaks, then yes you would see fluid in the boot. You would also loose fluid from the reservoirs. If the seal on the other end of the piston is leaking, it would affect the braking but you wouldn't loose fluid. It would just bypass the seal and stay in the system. At least this is what I understand. Rebuild kits for the Wilwood MC's are just over $20 and really easy to do. There's a snap ring in the end and everything comes out. The rebuild kits include every part except the outside housing. My opinion is that's what it's going to take.

[Lidodrip]

I suspect that brake drag is not the issue. I have experienced brake drag before, and the symptoms were different. The drag was not noticeable when first starting out, but after 20 minutes of driving at highway speed, the car would not coast properly and would essentially stop itself. The brake pedal was exceptionally firm and it took at least a half hour to cool before the wheels would freely spin. The wheels were also hot to the touch. I agree with others about the master cylinders - that is where I would direct my energy and time. Good luck.

James

[Nigel Allen]

Hoses swelling at the wheel cylinders?

[lewma]

UPDATE: So a quick update on the situation. Taking the board's advice I checked both brake master cylinders by pulling back the rubber boots. Both are dry. As an experiement, I also just sat in the driver seat, put my foot on the brake with standard pressure (with the engine off) and sat there for a minute or two. No "softening" occured. Finally. I looked into each reservoir and none appeared low (but I don't really have a reference here of where they all started, so hard to tell).

Is there another test I should be running to check the master cylinders?

My next test (unless the board has another idea) is to put the car up on the lift and spin each tire, see if any of the brakes are dragging.

Introduce heat into your test ?

[JohnK]

If the seal on the other end of the piston is leaking, it would affect the braking but you wouldn't loose fluid. It would just bypass the seal and stay in the system.

Interesting. I wasn't aware of this failure mode. I always assumed that if there was no visible fluid leakage then the MC's were fine. Thanks for this.

[Mike.Bray]

Interesting. I wasn't aware of this failure mode. I always assumed that if there was no visible fluid leakage then the MC's were fine. Thanks for this.

40+ years ago I had a 70-something Chevy truck that had been wrecked and "rebuilt" so it had a lot of issues. One of them was the brake pedal would slowly bleed down while sitting at a light. Looked everywhere and no leaks. Finally changed the MC and problemo solved. It was leaking internally, bypassing one of the seals on the piston. I had forgotten all about that until this thread jogged my memory.

Odds are that's what is happening with one of the Wilwoods.

[DOHCCOBRA]

When you checked the fluid in the master cylinders, did one seem darker than the other? As was mentioned above, you can have the inner seal in the master cylinder leak, which just allows the fluid past the piston back into the reservoir. Usually, when this happens, it will turn the fluid a bit darker from the rubber particles from the seal breaking down. This could happen worse with heat as the rubber softens, which could add to the apparent randomness of what you are finding.

[TrackDay17]

I’d. say it’s probably your master cylinder.
My motorcycle racing experience I’ve had a Suzuki master cylinder get soft no matter how much I bled them it would be soft enough to almost come back to the bar after a 8 lap race.
I replaced the master cylinder with a high quality aftermarket one and the problem went away.
Six months later Suzuki recalled my bike to replace the master cylinder.

I’m planning on getting Tilton masters for my MK5 build.

Anyone have part numbers for what I’ll need to order and from who to get them ?
I’ll be running a Blueprint 347 with hydraulic clutch and I also opted for the brake booster on my order.

[BrewCityCobra]

Everything you're describing is consistent with either air in the system or a failing MC. Absence any other leaks, e.g. at the calipers, brake lines, etc. Based on the intermittent failure, likely it's not air in the system. There are two seals on the Wilwood MC piston. If the one nearest the pushrod leaks, then yes you would see fluid in the boot. You would also loose fluid from the reservoirs. If the seal on the other end of the piston is leaking, it would affect the braking but you wouldn't loose fluid. It would just bypass the seal and stay in the system. At least this is what I understand. Rebuild kits for the Wilwood MC's are just over $20 and really easy to do. There's a snap ring in the end and everything comes out. The rebuild kits include every part except the outside housing. My opinion is that's what it's going to take.

Yeah, I'm getting that feeling as well. I keep trying to check other items but in the end I think it is just me trying to avoid accepting the inevitable. Looks like I have a good winter project ahead of me.

To say I'm disappointed in Wilwood right now is an understatement.

[BrewCityCobra]

Introduce heat into your test ?

I'm gonna do that tomorrow. Let the car idle for a while, apply pressure, see what happens.

[BrewCityCobra]

40+ years ago I had a 70-something Chevy truck that had been wrecked and "rebuilt" so it had a lot of issues. One of them was the brake pedal would slowly bleed down while sitting at a light. Looked everywhere and no leaks. Finally changed the MC and problemo solved. It was leaking internally, bypassing one of the seals on the piston. I had forgotten all about that until this thread jogged my memory.

Odds are that's what is happening with one of the Wilwoods.

Yeah, I think that is the ticket. Time to accept my fate, pull those bad-boys out, and rebuild 'em.

[OB6]

Yeah, I think that is the ticket. Time to accept my fate, pull those bad-boys out, and rebuild 'em.

I wouldn't bother rebuilding. Just get the Tiltons and call it a day. I don't believe the rebuild kits address bore defects, assuming that's what caused a breach in the seal(s).

[cv2065]

I wouldn't bother rebuilding. Just get the Tiltons and call it a day. I don't believe the rebuild kits address bore defects, assuming that's what caused a breach in the seal(s).

Might not be as easy as swapping out due to the brake line plumbing. I’d just replace the cylinder versus a rebuild as easy enough to get to on the brake side. As mentioned before, installing or removing the Tiltons on a buttoned up car is going to be more of a challenge.

[Mike.Bray]

Might not be as easy as swapping out due to the brake line plumbing. I’d just replace the cylinder versus a rebuild as easy enough to get to on the brake side. As mentioned before, installing or removing the Tiltons on a buttoned up car is going to be more of a challenge.

Good plan, it's only your life riding on it right?

Remember, with the balance bar if one MC fails you have zero brakes....

[Mike.Bray]

I’m planning on getting Tilton masters for my MK5 build.

Anyone have part numbers for what I’ll need to order and from who to get them ?
I’ll be running a Blueprint 347 with hydraulic clutch and I also opted for the brake booster on my order.

You can use the Tilton 75 series although Paul is reporting they're a little tight on the MKV. I used the 76 series which uses banjo fittings for the inlet and outlet.

As for size, I ended up with three different ones. Tilton can help you with sizing, just give them a call.

For the clutch, I'm using a Tilton 6200 series hydraulic throwout bearing with a 13/16" MC and it's perfect.

[Jeff Kleiner]

I used 76s on my Mk5.

Jeff

[OB6]

I used 75's on my Mk4 with minimal rework, but the body hadn't been installed yet, and my fuse panel is in the trunk. Even with the body on, I'd find a way to make the Tiltons work.

219884

[edwardb]

Good plan, it's only your life riding on it right?

Remember, with the balance bar if one MC fails you have zero brakes....

Every time I see this I cringe a little because some on here may not question it. Let's be clear -- without getting into the Wilwood vs. Tilton debate -- a failure of a single MC in a dual/balance bar setup is not good. The car needs to be parked immediately and the problem resolved before driving again. But multiple references (including Wilwood literature) describe the following if a single MC fails in a dual MC balance bar setup: (1) A failure in one circuit does not cause a loss of pressure in the other. (2) If one MC fails (e.g. zero or low resistance) the MC will go against its stop and the balance bar will put full force on the remaining MC. (3) pedal feel will obviously be affected. (4) definitely reduced braking power. (5) likely uneven braking due to the unbalanced front vs. rear bias, possibly even affecting how the car handles. None are good and even potentially serious. I'm not minimizing it. But not zero braking. There is a difference. Plus, although effectiveness is variable depending on the setup and situation, there's also the e-brake.

I'm not going to debate or argue (not that it matters that much...). This isn't me talking it's multiple references. Look it up.

[edwardb]

I wouldn't bother rebuilding. Just get the Tiltons and call it a day. I don't believe the rebuild kits address bore defects, assuming that's what caused a breach in the seal(s).


Might not be as easy as swapping out due to the brake line plumbing. I’d just replace the cylinder versus a rebuild as easy enough to get to on the brake side. As mentioned before, installing or removing the Tiltons on a buttoned up car is going to be more of a challenge.

Unless you are dead set to replace the Wilwoods with Tilton, trying a rebuild is easy and relatively inexpensive. Around $20 for the rebuild kit vs. around $100 for an all new MC. As I stated earlier, the rebuild kit comes with all new parts for the internals. Only thing not replaced is the housing. It's literally as simple as removing a snap ring, pull out the old assembly, push in the new, and replace the snap ring. Of course the bore of the MC could be defective (anything's possible) and I'd certainly examine it closely. I state all of this from personal experience. On one of my builds I had a brand new Wilwood MC start putting a few drops of brake fluid into the boot. I took it apart and found the outside O-ring was defective. It had a spot that wasn't filled. I suspect it was a short shot when it was manufactured. I posted pictures. I doubt Wilwood made it. Probably from a supplier. I'm not making excuses for it. But it was an easy fix and the build has gone thousands of uneventful miles since. This event is sometimes cited as me experiencing a Wilwood failure and lumped into the perception of an overall quality issue. Not sure that's completely warranted. But I'm sure others won't agree.

[Justin]

There are probably a LOT of Wildwood MCs sitting on shelves. Maybe some one will just gift you one. Easy swap.

[Nigel Allen]

Every time I see this I cringe a little because some on here may not question it. Let's be clear -- without getting into the Wilwood vs. Tilton debate -- a failure of a single MC in a dual/balance bar setup is not good. The car needs to be parked immediately and the problem resolved before driving again. But multiple references (including Wilwood literature) describe the following if a single MC fails in a dual MC balance bar setup: (1) A failure in one circuit does not cause a loss of pressure in the other. (2) If one MC fails (e.g. zero or low resistance) the MC will go against its stop and the balance bar will put full force on the remaining MC. (3) pedal feel will obviously be affected. (4) definitely reduced braking power. (5) likely uneven braking due to the unbalanced front vs. rear bias, possibly even affecting how the car handles. None are good and even potentially serious. I'm not minimizing it. But not zero braking. There is a difference. Plus, although effectiveness is variable depending on the setup and situation, there's also the e-brake.

I'm not going to debate or argue (not that it matters that much...). This isn't me talking it's multiple references. Look it up.

This is correct. Otherwise, as a practical example, during initial bleeding with both front and rear circuits full of air, it would not be possible to bleed one brake circuit, whilst the other has air in it, allowing the pedal to go to the floor .

[BornWestUSA]

I used 76s on my Mk5.

Jeff

Can you post a picture of the install??

[Mike.Bray]

Can you post a picture of the install??

Here's my 76 Tiltons on my MKIV. If I was doing it again I would run flex lines from the bulkhead fittings to the inlets.

219886

[OB6]

Unless you are dead set to replace the Wilwoods with Tilton, trying a rebuild is easy and relatively inexpensive. Around $20 for the rebuild kit vs. around $100 for an all new MC.

Fair point and I'm sure it works out for many (like it did for you), but given the choice of $20 plus some time or $100 and a little more time, I'll pick the latter any day knowing the Tiltons are demonstrably better components.

[Mike.Bray]

I know I'm going to get called crazy for this. Like a lot of us in my younger days I couldn't rub two nickels together and keeping a car or truck running was a real struggle at times. Now that I can rub at least one nickel together I want to avoid bad memories so there's basically three things I want a car to do.

Start when I turn the key. This means a good battery, good starter, and EFI

Go when I press the gas pedal and not die at lights. Again EFI helps with this.

Stop when I hit the brakes. I choose to not use dodgy components in my brake systems for this reason.

Yeah I know, crazy isn't it?