spongy brakes, pedal goes to floor

I'm having a big problem with my brakes. The pedal is soft/squishy, and slowly sinks to the floor.
What I've read on the internet is that soft/squishy brakes is usually caused by air in the lines. But a pedal that sinks to the floor is caused by a bad master cylinder or a leak.
I am using the 4 stock mustang calipers that came off my donor 1996 GT. It has two piston PBR's in the front. This spring I bought powerstop Z26 pads, which are supposed to have higher friction. I thought they might be better than the stock pads I bought at auto zone when I built the car 10 years ago.
First problem. I could not retract/turn the rear caliper pistons into the calipers, probably because they were 30 years old. I bought 2 new rear calipers at autozone. I added new DOT 3 brake fluid (that I bought last month) ,and bled all four calipers. I was careful not to let the fluid level in the reservoir get too low. I "adjusted" for the parking brake by pulling the lever about 10 times (which is how I've read you're supposed to adjust it.) I could not get a firm pedal, and it would sink to the floor.
My research on the internet led me to believe that the master cylinder had an internal leak. I purchased a new Dorman master cylinder (M39637 for an 87-93 mustang), which is what factory five told me to use when I first built the car. I bench bled the MC, following the instructions in the box, installed it, then bled all four calipers. No change; it's still soft, and sinks to the floor.
I don't see any fluid leaks. there are no bubbles when I bleed the brakes. I've used almost a full quart of new brake fluid.
What am I missing? Could the new rear calipers or the new MC be bad? Could there be something wrong with the new carquest brand fluid?
Thanks for your input.

Everything you're describing is either air in the lines or a bad master cylinder. Anything else would have fluid coming out somewhere. I assume this worked in the past? Anything changed other than it stopped working?

Is it possible the have the rear calipers on the wrong side? Bleeder needs to be above the brake line coming in. Easy to do on those rear calipers. Another thing to consider is whether your pedal is retracting all the way. Make sure there is some free play between the pedal and the 3/4 bar in front of it.
Mike

Thanks Mike and EdB
Mike, I just double checked the calipers. I've got them on the correct sides, with the bleeder above the brake line inlet. Also, I adjusted the master cylinder push rod so there is a little free play, so I'm pretty sure it is retracting all the way.

EdB, the brake pneumatics worked ok last year. I just wanted to try better pads. I couldn't get the new pads in because I couldn't compress the rear caliper pistons, so I bought new calipers. (The calipers pistons screw in and out, and they wouldn't turn. Also, I figured they are 30 years old, rusty and dirty. It's nice to work with clean new parts once in a while.) I bought the new MC because I've read that a pedal that goes to the floor is a symptom of a bad MC.

I'll buy some more brake fluid and try bleeding them some more.

This all brings to mind a couple cliches, like I should have left well enough alone, and if it ain't broke, don't fix it.

If you have resistance in the pedal but then it slowly bleeds down that is usually the MC leaking internally, fluid bypassing the piston. Air in the lines is typically just spongy.

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Originally Posted by Mike.Bray

If you have resistance in the pedal but then it slowly bleeds down that is usually the MC leaking internally, fluid bypassing the piston. Air in the lines is typically just spongy.

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Hey Joel,

Unfortunately I completely agree with Mike here. Let me know if you need any help replacing it. Happy to stop by and give you a hand.

Good luck

Thanks Mike and Jesse, I'm going to try bleeding the system again. Maybe the air bubbles are closer to the MC, and they're just not getting pushed out the rear calipers? The truth is, in my 72 years on this planet, I've only had to bleed brakes a handful of times, and each time I say to myself, Am I doing this right?
It will be frustrating if the new MC is leaking internally.

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Originally Posted by Joel Hauser

Thanks Mike and Jesse, I'm going to try bleeding the system again. Maybe the air bubbles are closer to the MC, and they're just not getting pushed out the rear calipers? The truth is, in my 72 years on this planet, I've only had to bleed brakes a handful of times, and each time I say to myself, Am I doing this right?
It will be frustrating if the new MC is leaking internally.

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What master cylinder do you have?

You could try gravity bleeding since the calipers are lower than the MC. Get a couple of these https://www.amazon.com/dp/B000CMDP44...sin_title&th=1

Let it gravity bleed for a few hours topping up the fluid reservoir as needed.

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Originally Posted by Joel Hauser

I'm having a big problem with my brakes. The pedal is soft/squishy, and slowly sinks to the floor.
What I've read on the internet is that soft/squishy brakes is usually caused by air in the lines. But a pedal that sinks to the floor is caused by a bad master cylinder or a leak.
I am using the 4 stock mustang calipers that came off my donor 1996 GT. It has two piston PBR's in the front. This spring I bought powerstop Z26 pads, which are supposed to have higher friction. I thought they might be better than the stock pads I bought at auto zone when I built the car 10 years ago.
First problem. I could not retract/turn the rear caliper pistons into the calipers, probably because they were 30 years old. I bought 2 new rear calipers at autozone. I added new DOT 3 brake fluid (that I bought last month) ,and bled all four calipers. I was careful not to let the fluid level in the reservoir get too low. I "adjusted" for the parking brake by pulling the lever about 10 times (which is how I've read you're supposed to adjust it.) I could not get a firm pedal, and it would sink to the floor.
My research on the internet led me to believe that the master cylinder had an internal leak. I purchased a new Dorman master cylinder (M39637 for an 87-93 mustang), which is what factory five told me to use when I first built the car. I bench bled the MC, following the instructions in the box, installed it, then bled all four calipers. No change; it's still soft, and sinks to the floor.
I don't see any fluid leaks. there are no bubbles when I bleed the brakes. I've used almost a full quart of new brake fluid.
What am I missing? Could the new rear calipers or the new MC be bad? Could there be something wrong with the new carquest brand fluid?
Thanks for your input.

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Just experienced this with my 70 GT350 convertible, it's a classic example of a bad master cylinder. I'm hoping unlike my issues, that you are using a DOT 5 or 5.1 fluid vs my original DOT 3 (now switching to DOT 5 or 5.1 once my replacement arrives today or tomorrow). Once you bench bleed the new master and install it, I highly recommend that you power flush the entire brake system front to rear. Replace all fluid, finish with a bleed of the system, and be done with it.


Bill S.

I was surprised how much fluid I ran through my system while bleeding the lines. I was impatient and thought I had all the bubbles out…I was wrong!

thanks Mike, Max and Mr. M.
Mike, I used the MC from an 87-93 mustang. It has 3 ports. One to the front driver, one to the front passenger, and one to both rear wheels.
Max and Mr.M, I'm thinking that maybe I'm being a little impatient, too. I'm going to buy another quart of DOT3 and try bleeding it some more and see how that goes.
Keeping the MC filled while bleeding it is kind of a pain; it's crammed in under the fender. it was a lot easier 10 years ago, before the body went on (and I was a lot more flexible.).
Joel

Air trapped in the m/cyl? Did you prefill the new master cylinder and bench bleed it before installing? Doing that can save a lot of grief.

I used a Jeep m/cyl as the bore size was larger than the 87-93 Mustang and was recommended for those who added rear disc brakes. Rear discs were not used on those year Mustangs, and calipers needed more fluid to be moved, than drum brake rear cylinders.

Thanks AC Bill. Yes, I bench bled the new MC, following the instructions that came with it. After sealing the ports with the screw-in plastic plugs, I filled the reservoir. I pushed in the piston a few times, about one inch each push, using a drum stick. After 3 or 4 times, and topping off the reservoir, I could only push it in about an 1/8 of an inch, which is what the instructions said is supposed to happen. Now that I'm thinking about, maybe that suggests that there isn't an internal leak?
When I was building the car I had asked someone at Factory Five what MC to use, and they told me to use one from an 87-93 mustang. You're probably right, I probably should have used one with a larger bore.

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Originally Posted by Joel Hauser

thanks Mike, Max and Mr. M.
Mike, I used the MC from an 87-93 mustang. It has 3 ports. One to the front driver, one to the front passenger, and one to both rear wheels.
Max and Mr.M, I'm thinking that maybe I'm being a little impatient, too. I'm going to buy another quart of DOT3 and try bleeding it some more and see how that goes.
Keeping the MC filled while bleeding it is kind of a pain; it's crammed in under the fender. it was a lot easier 10 years ago, before the body went on (and I was a lot more flexible.).
Joel

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Joel,

I have a bleeder kit. One of the nice features is it has a reservoir that functions like a hamster water container. You fill it, install it, open the valve then it automatically keeps the master cylinder topped off. Never have to worry about it. You can borrow it, it makes it much easier.

I also find brake bleeding is much easier with 2 people if you can. Again, happy to come by and lend a hand.

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Originally Posted by Joel Hauser

Thanks AC Bill. Yes, I bench bled the new MC, following the instructions that came with it. After sealing the ports with the screw-in plastic plugs, I filled the reservoir. I pushed in the piston a few times, about one inch each push, using a drum stick. After 3 or 4 times, and topping off the reservoir, I could only push it in about an 1/8 of an inch, which is what the instructions said is supposed to happen. Now that I'm thinking about, maybe that suggests that there isn't an internal leak?
When I was building the car I had asked someone at Factory Five what MC to use, and they told me to use one from an 87-93 mustang. You're probably right, I probably should have used one with a larger bore.

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I have never heard of bleeding a MC with the ports plugged. Over the past 50 years all the MCs I have purchased came with fittings to put in the ports with clear tubing that you loop back into the reservoir. You then pump the MC until there are no bubbles. It will take a lot more than 3 or 4 times to get all the air out. Plus you need to stroke the piston it's full travel.

Note: you can you a couple of pieces of brake line to loop back into the reservoira.

Dgc333, the instructions for the bleeding the M39637 are at https://static.dormanproducts.com/do.../m39637_is.pdf I think I followed them carefully. Like others have suggested, I'm going to continue bleeding the system, and hope the MC I purchased isn't defective.

Jesse, thanks so much for the offer to help out. You're the best. My wife has reluctantly agreed to help me a few more times. But if I can't resolve it on my own I know who to call.

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Originally Posted by Joel Hauser

...When I was building the car I had asked someone at Factory Five what MC to use, and they told me to use one from an 87-93 mustang. You're probably right, I probably should have used one with a larger bore.

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The '87-'93 disc/drum master cylinder does not work well with the rear discs. When using the Mustang front and rear discs the ideal master cylinder is the 15/16" bore version used on the 1994-1995 Mustang Cobra.

Jeff

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Originally Posted by Jeff Kleiner

The '87-'93 disc/drum master cylinder does not work well with the rear discs. When using the Mustang front and rear discs the ideal master cylinder is the 15/16" bore version used on the 1994-1995 Mustang Cobra.

Jeff

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Thanks Jeff. I don't have the energy or skill to install a 94-95 MC. I looked at those on line, and I think they have only 2 ports. I would have redo all the brake lines, and use a proportioning valve, which I think would be a major pain with the body on. also, while the brakes were not great in the past, they did work well enough for my level of driving. I'm going to keep trying to bleed them. Hopefully, the problem is air in the line.

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Originally Posted by Joel Hauser

Thanks Jeff. I don't have the energy or skill to install a 94-95 MC. I looked at those on line, and I think they have only 2 ports. I would have redo all the brake lines, and use a proportioning valve, which I think would be a major pain with the body on. also, while the brakes were not great in the past, they did work well enough for my level of driving. I'm going to keep trying to bleed them. Hopefully, the problem is air in the line.

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No you do not use a proportioning valve, you would only need to add a tee fitting and one new line

Quote:

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Originally Posted by Joel Hauser

Dgc333, the instructions for the bleeding the M39637 are at https://static.dormanproducts.com/do.../m39637_is.pdf I think I followed them carefully. Like others have suggested, I'm going to continue bleeding the system, and hope the MC I purchased isn't defective.

Jesse, thanks so much for the offer to help out. You're the best. My wife has reluctantly agreed to help me a few more times. But if I can't resolve it on my own I know who to call.

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Regardless of what Dorman is telling you I would get some short pieces of brake line and put them in the ports looped back up into the reservoir. Stroke the MC it's full range until you get no bubbles. Another option is to get a pressure bleeder and force fluid from the reservoir through the system to the caliper bleeders.

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Originally Posted by rich grsc

No you do not use a proportioning valve, you would only need to add a tee fitting and one new line

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Rich is correct.

Jeff

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Originally Posted by Dgc333

Regardless of what Dorman is telling you I would get some short pieces of brake line and put them in the ports looped back up into the reservoir. Stroke the MC it's full range until you get no bubbles.

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Agree, unless you're moving fluid through the MC and ports I don't see anyway you're going to get the air out.