Rear brakes not working

[VKannan]

Hello,

I have a MK4 complete kit with a rear end built by MPS auto salvage that included pre-assembled mustang GT rear brakes. I’m running the standard complete kit Wilwood pedal box and master cylinder. I’ve bled the brakes twice now and there are no more bubbles coming out. When I push the brake pedal down I am unable to turn either the front or rear wheels by hand, so they are working to some extent. But when I have the car up on jackstands and turn the engine on and put it in gear, and let it idle in first gear, pushing on the pedal fails to stop the rear brakes. It slows them down some but they just keep turning. What are the possible causes of this? I’m not sure where to begin troubleshooting.

I suppose it could be a bleeding issue, but there’s no brake fade when I hold the pedal down, and no more bubbles are coming out, so I think this is less likely.

It could also be the brake bias, here’s a picture of how my setup currently looks, how do I go about adjusting the threaded rod for brake bias after the pedal box has been installed? It’s very difficult to turn that rod by hand, am I missing something? I don’t want to grab it with channel locks and risk damaging the threads.

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Any other thoughts on where I could look for problems? I have read some thing about a possible “self adjusting“ mechanism involving the emergency brake? I have not yet installed my emergency brake. Not sure if that has anything to do with us.

I suppose I should also mention that I don’t know for sure if the front brakes are working properly, since I can’t turn either front or rear by hand but I know that with enough force from the motor the rear brakes are not working. Only way to figure out if the front brakes are working is to drive it, I suppose?

[CraigS]

You adjust the front to rear bias by making the exposed threads on the end of the shaft longer on the side you want to add bias to. In your case make the left side longer. BTW when looking at this ignore the nut on the right side. Count the threads included inside the nut. BTW you may also need to look at the gap pedal to clevises. Yours looks to be too tight. Read here and look at Fig 3 & 4.
https://www.wilwood.com/PDF/DataSheets/ds252.pdf
Also, what is stopping your pedal from coming toward you when you take your foot off of it? I ask because I wonder if you have enough overall pedal travel to fully actuate the calipers.

[Mastertech5]

The Mustang park brakes are self adjusting after adjusting the cables properly. Disc brakes are all self adjusting. You should have about 2" MAX pedal travel before the pedal gets hard. Try turning the pushrods out of the clevises some as Craig's suggests.

[rich grsc]

The Mustang park brakes are self adjusting after adjusting the cables properly. Disc brakes are all self adjusting. You should have about 2" MAX pedal travel before the pedal gets hard. Try turning the pushrods out of the clevises some as Craig's suggests.
BUT, on a new pad install if the pistons where turned all the way in, the parking brakes need to be cycled to remove the extra clearance in the calipers.

[Mastertech5]

BUT, on a new pad install if the pistons where turned all the way in, the parking brakes need to be cycled to remove the extra clearance in the calipers.

Sometimes but usually with new pads there isn't that much space to take up and a few pumps of the pedal will do the trick. From my experience most people rarely use their park brake (state inspections) and the pistons move out as the pads wear just fine. I always did activate the park brakes on a brake job though to be thorough.

[VKannan]

How do I turn that threaded rod? There’s threads on both ends and I’m not sure how to turn it because it won’t go by hand.

Similarly, how do I increase the gap in the clevises once the pedal box is already installed? I can’t back the bolts coming from the master cylinders out of the clevises to free them up because they stop when they fully compress the cylinders.

The pedal is stopped by the master cylinder rods engaging, which happens a fraction of an inch before the pedal hits the frame.

Thank you!


You adjust the front to rear bias by making the exposed threads on the end of the shaft longer on the side you want to add bias to. In your case make the left side longer. BTW when looking at this ignore the nut on the right side. Count the threads included inside the nut. BTW you may also need to look at the gap pedal to clevises. Yours looks to be too tight. Read here and look at Fig 3 & 4.
https://www.wilwood.com/PDF/DataSheets/ds252.pdf
Also, what is stopping your pedal from coming toward you when you take your foot off of it? I ask because I wonder if you have enough overall pedal travel to fully actuate the calipers.

[Ducky2009]

..... But when I have the car up on jackstands and turn the engine on and put it in gear, and let it idle in first gear, pushing on the pedal fails to stop the rear brakes. It slows them down some but they just keep turning. What are the possible causes of this?

If you're in gear with the clutch out, you'd have to stall the engine to completely stop the wheels from turning. Try pushing in the clutch then braking hard. How fast do the wheels stop?

[CraigS]

How do I turn that threaded rod? There’s threads on both ends and I’m not sure how to turn it because it won’t go by hand.

Similarly, how do I increase the gap in the clevises once the pedal box is already installed? I can’t back the bolts coming from the master cylinders out of the clevises to free them up because they stop when they fully compress the cylinders.

The pedal is stopped by the master cylinder rods engaging, which happens a fraction of an inch before the pedal hits the frame.

Thank you!
I am concerned that it won't turn by hand. I was thinking that the pedal might be stopped by that frame tube and so keeping slight pressure on the bias adjuster shaft. Are your SURE there is some clearance to the frame tube? That brass colored nut on the right end of the shaft is a jam nut to lock it. If you loosen that nut some the shaft should be able to be turned by hand. In the link above read the bottom 3 paragraphs of Pg2 and all of Pg 3 for some info on pedal setup. Also I just looked at your picture again and see on the right side clevis a slotted shaft w/ a jam nut. I haven't noticed one of those before and it looks like a lock for the bias adjusting shaft.

[VKannan]

Well, one question answered! You are correct, in addition to the jam nut on the side there’s also a jam nut on the top that I had to loosen in order to freely turn the threaded rod by hand. It now moves back-and-forth quite easily, allowing me to adjust the bias.

Two questions:
1) does adjusting the threaded rod such that there are more threads sticking out on side X mean that side X has a stronger bias, and will see more force? Or is it the opposite?

2) Ducky2009 raises an interesting point. Am I even supposed to be able to stop the rear axle from turning with the engine idling and the transmission in first gear? That should theoretically stall the engine, right? Are the rear brakes supposed to be strong enough to stall the engine when it is running and the car is up on jackstands? If that’s not the case then my whole premise that the rear brakes are not working properly is wrong.


I am concerned that it won't turn by hand. I was thinking that the pedal might be stopped by that frame tube and so keeping slight pressure on the bias adjuster shaft. Are your SURE there is some clearance to the frame tube? That brass colored nut on the right end of the shaft is a jam nut to lock it. If you loosen that nut some the shaft should be able to be turned by hand. In the link above read the bottom 3 paragraphs of Pg2 and all of Pg 3 for some info on pedal setup. Also I just looked at your picture again and see on the right side clevis a slotted shaft w/ a jam nut. I haven't noticed one of those before and it looks like a lock for the bias adjusting shaft.

[Hoooper]

Yes, you should easily be able to stall an engine idling in gear by using the brakes

[VKannan]

Ok thanks, that’s what I needed to know to go forward. There must be a problem that I need to identify.

Still wondering about this one:

1) does adjusting the threaded rod such that there are more threads sticking out on side X mean that side X has a stronger bias, and will see more force? Or is it the opposite?


Yes, you should easily be able to stall an engine idling in gear by using the brakes

[CraigS]

Yes make the threads longer on the side you want the bias moved to. I have to admit that I have never tried your experiment (stalling engine by applying brakes) w/ brand new unbedded pads, but in the back of my mind I am wondering if this is normal. It may well be normal and they will be fine once driven on the street. Have you looked at the angle of the bias adjuster when you push hard on the pedal? See the top of page 4 in the link above. Also, while I am somewhat hesitant to question Wilwood, I have seen other recommendations regarding the pushrod length. They wanted the bias adjuster shaft pushrod to be as close to a 90deg angle to the pushrods as possible at full brake pedal pressure. Different opinions from different trusted sources. IE the Wilwood diagram would be changed to this;
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Probably not a huge difference, just something to consider..

[totem]

You adjust the front to rear bias by making the exposed threads on the end of the shaft longer on the side you want to add bias to. In your case make the left side longer. BTW when looking at this ignore the nut on the right side. Count the threads included inside the nut. BTW you may also need to look at the gap pedal to clevises. Yours looks to be too tight. Read here and look at Fig 3 & 4.
https://www.wilwood.com/PDF/DataSheets/ds252.pdf
Also, what is stopping your pedal from coming toward you when you take your foot off of it? I ask because I wonder if you have enough overall pedal travel to fully actuate the calipers.

Reference to Figure 4: DIMENSION A + B ADDED TOGETHER MUST BE IN THE .20" - .25" RANGE

According to OP picture, it might not be the case. The balance bar might be binding, preventing proper balance to the rear once front pressure is apply.




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[VKannan]

A question and a few updates:

First, I ordered a set of parking brake cables for a 1992 mustang since my rear end is using mustang GT brakes, they have a spring wrapped around the cable at one end, I don’t see that spring in the pictures in the instruction manual, is it supposed to be there? What do I do with it?

Now the updates:

I’m wondering whether or not the self adjusting screw that works when the emergency brake is pulled is part of the problem, so I’m trying to get those going. I have read other posts saying people experienced similar symptoms before their emergency brake had been installed.

Additionally, it’s possible that my master cylinder is too large of a bore for the SN 95 mustang GT rear brakes, it’s a 0.75 inch bore, and apparently factory five used to send out a 0.625 inch master cylinder for those brakes. So if the parking brake adjustment doesn’t do it then I’m going to try swapping out the master cylinder.

Finally, I measured the distance between clevis on the pedal and it’s exactly 0.25 inches, so I don’t think I’m binding the balance bar, that’s probably not the issue.

Continued thanks to everyone who has been weighing in on this thread.

[Mastertech5]

Those cables are for drum brakes. You need to get Mustang cables for disc brakes. I think I got mine for a 2003 Mustang.

[F500guy]

That change to the master cylinder would not likely "make" or "break" a system, it would only fine tune the pedal feel.
Smaller master cylinder will require more pedal distance with less force at the pedal
Larger Master cylinder would need more force to operate, but the pedal would not travel as far to move the pistons at the caliper.

[CraigS]

Yes a 5/8 MC was standard back in the day. Sorry to ask this so late but what brake pads do you have? Compound can make a huge difference.

[VKannan]

Good question, I am not sure what brake pads MPS auto salvage uses in its factory five specific rear end. I’ll have to give them a call.

I did get my emergency brake figured out with some universal cables and creativity, and it also stops the axle from turning at all when I attempt to rotate the wheels by hand, but fails to stop it from spinning completely when the car is in gear. Both pushing on the brake pedal and engaging the emergency brake slow it down, and I can hear the engine working a bit harder as if it is under threat of stalling, but neither of them completely stop the axle from spinning. I would imagine that this means if there is even a problem, it is at the caliper itself, since they act three different mechanisms, correct? Though it seems several people both on this forum and off think that there may not be a problem and that I need to just go kart the car to let the pads settle and see where things are at.

[Mastertech5]

Don't judge until you bed the brakes. It makes a big difference. 10 medium stops from about 40 Mph then a couple hard stops but let them cool down some in between.

[CraigS]

Well I wouldn't bet $ but I bet that whatever pads are in there are the cheapest they can get which would work fine in a car w/ power brakes. I agree w/ Mastertech5, I would put this Q on the shelf until you can drive it. And I am not even sure that gokarting it will do the deed. His recommendation is kind of hard for most people to do on the street w/ tags.

[VKannan]

Update for everyone that weighed in - it was just the pad seating. After 10-15 runs with hard braking I come to a very firm stop and am able to lock up the tires. This was a good learning experience!

[CraigS]

Yeeeehawwww! I am really glad it worked out for you.