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Discussion Starter · #1 ·
Car is an 83 Mustang with Aerospace rear disks and stock discs up front.

The rear brakes are working fine, but the fronts are keeping pressure. After a day or two, they will bleed enough pressure to move freely again, but one good jump on the pedal and they are stuck again.

I suspected the wrong pushrod length, but I pulled the MC off the firewall completely and the pressure is still there.

then I though the Line Lock might be hanging up, but if I crack the line loose from the MC and bled a touch of fluid off, the piston retracts, so it appears to be a problem with the MC.

It's a stock replacement reman unit that I just recently bought.

I've checked for kinks in the lines or sharp bends and there is nothing. Could it just be a "bad" master cylinder? Seems odd. Oh yeah - and no prop valve or anything.

Also, the front brakes come off the REAR of the MC (large resevior) and the rear brakes from teh FRONT of the MC (small resevior) Does that sound right?

Any other ideas? Thanks.
 

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My car had a VERY similiar problem, and it turned out to be the front calipers were frozen on the sliders. You could crack the bleeder, and compress the piston, and all was well until 3 or 4 pumps, then locked up tighter than a drum. You may want to start there. You have the lines hooked up correctly, and the proportioning valve you will need to lower the pressure going to the rear disk to prevent them from locking up when you get on the brakes hard. Just Fyi. Hope this helps

Bill
 

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Car is an 83 Mustang with Aerospace rear disks and stock discs up front.

The rear brakes are working fine, but the fronts are keeping pressure. After a day or two, they will bleed enough pressure to move freely again, but one good jump on the pedal and they are stuck again.

I suspected the wrong pushrod length, but I pulled the MC off the firewall completely and the pressure is still there.

then I though the Line Lock might be hanging up, but if I crack the line loose from the MC and bled a touch of fluid off, the piston retracts, so it appears to be a problem with the MC.

It's a stock replacement reman unit that I just recently bought.

I've checked for kinks in the lines or sharp bends and there is nothing. Could it just be a "bad" master cylinder? Seems odd. Oh yeah - and no prop valve or anything.

Also, the front brakes come off the REAR of the MC (large resevior) and the rear brakes from teh FRONT of the MC (small resevior) Does that sound right?

Any other ideas? Thanks.
Yes your master is hooked up correctly. Most of those master cylinders have residual check valves built into the port itself. Make sure that the valve is #1) clear or you may even end up taking the valve out all together. If by chance your master doesnt have a residual built in then it sounds like you have a bad master cylinder.
 

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Discussion Starter · #4 ·
Thanks Bill.

that was my first thought too. I took the sliders out and it took a big rubber mallet to get the caliper off still :D lol. I put a giant C clamp on it and a 12" wrench on the clamp and I couldn't push the piston in at all. Only way it retracted was when I cracked the bleeder on the caliper. thenit went in easy which amakes me thingk it's not the piston.

I'm going out now to pull the calpers apart and clean them a bunch, but since it does move with pressure off - that's what had me look upstream.

BTW - Yeah, I'm going to get a prop valve, but I wanted to get everything working correctly before throwing another variable at it. Thanks.
 

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Discussion Starter · #8 ·
Not a bad thought on the lines - I'll check/replace them as well.
Joel - thanks also. No residual valve that I can tell.

Just got the pistons out - holy crap, literally. maybe they were just stuck. lots of crud on the pistons and the caliper bore had caked in gunk (like that tar you get when gas sits a long time in a tank).

it took 80 psi of air to get one of them out and 50 psi on the other. After I cleaned it, I tapped the inside of the piston with the handle of my rubber mallet to center it, and then I was able to push it in easily by hand. 80 psi out and slides in by hand after.

Maybe whatever gunk was in the bore made it's way up the line and clogged the MC too, but right now, it looks like maybe they were just stuck.

Keep the other ideas coming though.
 

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Discussion Starter · #11 ·
Right now, they are disconnected! lol!
 

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Discussion Starter · #13 ·
actually, at Memphis I really don't even need the brakes or chute. Staging is just a bitch without them :p
 

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I have seen a bunch of bad master cylinders in the last 2 years. The first indication of a problem was that it was a bitch to bleed the brakes. After pumping for an hour , checking the pushrod length thinking it was too long it was a problem in the master itself. The piston was not returning back past the conpensation port and was keeping fluid trapped in the chamber. Many cars went out and the brakes locked up on the pass as the fluid heated up in the calipers or they were sitting on return road and the car would not move until they cooled off. Those who had a brake pressure guage saw the guage sitting there at 300+ psi after the run and knew the problem was brakes. I had lots of calls on why my trans felt sluggish on return road , so I can say from experience that there is problems out there with performance master cylinders.

Hutch
 

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Discussion Starter · #15 ·
Hutch, this isn't a performance MC at all - just a parts store reman replacement, but that pretty much describes everything perfectly. We did have a lot of trouble bleeding the system and every time it seemed good, after a couple pumps, it would be soft pedal again. finally got to where the pedal stayed firm.

Car always run good until after the burnout. It kept trying to stall and I had a hell of a time trying to stage the car. I guess the reason was I never got on the brakes hard except when I set the LL to do my burnout. After that I was pretty much trying to push the front brakes.

The car sounded so dead until near the very top of 1st gear and my 60 was a tenth and a half slow. I kept thinking it was retarding timing somehow as it felt about 20* retarded. Full-on front brakes would do that too I guess! :D

'm going to bleed it tonight with the newly free'd up front calipers and see ow it does. if it's still hanging, I think it's definitely the MC.
 

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Discussion Starter · #17 ·
Lines apparently were fine, but I picked up some new ones and put them on today. I assume these were still stock, so after 25 years they should have been swapped for safety anyway.

Bled the brakes and they still stuck after putting the pedal down.

Pulled the MC off and dumped the fluid and put an air nozzle on the port. Got NOTHING back in the reservoir. checked my old MC and another one I had with bad piston and both had air back to the reservoir. So, either this one DOES have a residual valve built in somewhere or it's a bad MC.

I used an old MC that I had new pistons in (long story) and bench bled it and put it in. A quick bled and NOW I have brakes when I need them and no brakes when i don't. 8)

Fired it up and tried the linelock in the garage but it's raining and the floor is wet under the front tires. wanted to see if it held but it either didn't or (more likely) it was just pushing the front tires across the floor a few inches.

Thanks for the input everyone.
 

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I am having this exact same problem right now.....but I have a brand new Wilwood brake kit on mine, with brand new stainless lines, and a Autozone reman master cylinder for a disc brake car the same year as mine...I have tried everything I Ncan think of and done exactly as he did by unbolting the mc from firewall thinking it was the push rod..still locked up...but only one side...You guys think it would be the MC on mine too? I just bought it, but at 20 bucks it may be worth trying a different one...How can I determine if it is bad? All the Wilwood stuff is brand new.
 

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Discussion Starter · #19 ·
pulling the MC and putting air in the ports confirmed it to me that mine had a residual valve built in to it somewhere. Very odd since the stock ones don't and since all Mustangs had the MC up high on the firewall, I don't see any reason why and MC for that model would ever have one, but this one did.
 

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A couple years ago a friends dragster had this problem. Long story short, the seals got swelled up from contamination and apparently didn't allow the hydraulic system to "vent" to the reservoir when the pedal was released. Replaced the MC and all was good again. I've had brake lines do this on numerous occassions also.
 
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