Hey guys Im sure this topic has probably been beaten to death around here but I cant seem to find a result when I search. I completly replaced the brake system in my 67 firebird, new everything. Ive spent plenty of time bleeding the brakes and I have a nice firm pedal when the car is not moving. Occassionaly, usaully when I get to around 30 mph the pedal will get extremely soft and just go right to the floor, the brake warning light will come on, and the car will slowly come to a stop and sometimes the rear wheels will lock up, as I leave skidmarks in my pants. Then once I stop I will pump the pedal a couple of times and its firm again and it will be O.K. for a little while and then happen again. I really want to get this car running for summer but I cant drive it around anymore without trusting my brakes. Ive already cracked all the fitting with the pedal down but I didnt seem to get any air. Do I need to just continue bleeding the breaks? Is it just that I have air in the front causing the warning light to come on? Or could something be not functioning properly like the distribution block or MC? I do plan on having it pressure bled soon anyway, could this be the solution? Thanks.

My guess....

Your brakes aren't fully releasing...just the slightest bit of pressure on them. So the calipers heat up, which boils the fluid. Probably puts air in the system. That causes the brakes to apply pressure to the rotors, so they car comes to a slow stop.

When the system cools down, pressure is released and you can drive until the calipers heat up again.

Boiling fluid would introduce air and cause a soft pedal.

Did you take the master cylinder off during the brake swap? If so, maybe check the snap ring that holds the piston in place. If that's bent just a bit, the piston in the M/C won't fully release and will cause the problem.

I had the same problem on my Cobra replica. I know...it was a ford, but brakes are brakes.

That would make sense because when I first put the disc brake kit on the pads were sticking to the rotors, I couldnt even push it so I just drove it around for a bit and they eventually stopped sticking, is that sign that the brakes arent fully releasing? How long does it take to boil fluid because sometimes this will happen 30 seconds after leaving my driveway?

The MC is brand new, I actually had to send one back because the rear seal was busted, but before it broke I had the same problem.

bad master IMO.

I'm with Steve about a bad MC. Put a new MC on immediately. Bench-bleed it prior to installation.

Checking for dragging brakes is super-easy. Just jack up the car and see if you can spin the tire(s) by hand. There will usually be a slight brake drag, especially with disc brakes. If one (or more) seems hard to turn, then investigate further.

However, I still bet that it's a bad MC..................

Are you sure you're bleeding the brakes properly? Did you bench-bleed the MC prior to install?

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disc or drum?

Its got front disc brakes, and there is no rubbing. I did bench bleed the MC prior to installing it but I took it off anyway today and re-bench bled it and I managed to work out a couple of bubbles. Im going to re-install it, and bleed the system this time using speed bleeders, and Ill see how it works out. If it is bad MC what other signs are there to look for and what part of it do you guys think would be broken? Thanks.

I..........If it is bad MC what other signs are there to look for and what part of it do you guys think would be broken?....


The MC has a few internal rubber seals that contain the fluid and direct the pressure out to the appropriate line (front or rear). The seal(s) can leak (bypass) internally, thus you will not lose fluid nor will you see any visible signs of leakage.

A classic test for a MC goes like this - First, pump pedal once or twice rapidly and it should be high and firm. Now release pedal and then gently re-apply steady pressure to the pedal. Hold the pressure and see what happens. If the pedal slowly goes down to the floor, then this indicates internal seal leakage and the MC would need to be rebuilt or replaced. In the old days, we regularly rebuilt them and I had tons of the seals on hand. We would inspect/hone out the bore of the MC and replace the seals.

Of course, this test assumes the brake system has been properly bled. If the system has lots of air in it, then you may get false positive indication of a bad MC.

Hope this helps. ;)

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