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A little brake help please

crause

Well-known member
Joined
Sep 4, 2003
Messages
266
Location
Louisiana
Corvette
75 Convertible-steel blue / 2000 lt. pewter conv.
Quick description of what I did: My 1975 Corvette front calipers were leaking so, replaced both front calipers, with new pads and new GM rotors. Installed new master cylinder. I bled the system and was satisfied with the braking function. Took the car on three local outings over about a 3 week period and all good. The car sat for about a week and I needed to move it from its parking location on my driveway. Started right up and when I went to put by foot on the brake pedal to shift gears, the pedal went all the way to the floor. Didn't feel like it would build up pressure when I pumped it. I didn't have time to work on it right then so I very slowly moved the car and had only very minor braking when the pedal would hit the floor (brake light came on the first time it went to the floor).
Opened the master cylinder expecting to find the fluid gone but both were full (front was about 1/8" down). When I found some time to look at it again, I noticed that if I pumped the pedal with the engine off, the pedal would build up but with pressure it would slowly begin to fall. With the engine running, it was almost impossible to build up any pedal and would almost immediately go to the floor. No sign of leaking brake fluid anywhere and don't know quite where to start. I hate to just start replacing parts but I am thinking that I may have gotten a defective master cylinder and the fluid is getting past the piston seals somehow. This was a new master cylinder not a rebuilt.
If anyone has had a similar situation and can offer some insight I would greatly appreciate it. :confused
 

crause

Well-known member
Joined
Sep 4, 2003
Messages
266
Location
Louisiana
Corvette
75 Convertible-steel blue / 2000 lt. pewter conv.
Did you bench bleed the master cylinder before installing it?

yes, I followed the bench bleeding instructions that came with the new master cylinder.
 

crause

Well-known member
Joined
Sep 4, 2003
Messages
266
Location
Louisiana
Corvette
75 Convertible-steel blue / 2000 lt. pewter conv.
Did you measure the lateral runout after installation? If it's over .008" or so, air can be drawn into the caliper past the seals. Try bleeding both calipers as a first step.

Well, sort of; like i said before, I purchased original GM brand rotors. I also purchased a run out gauge and a magnetic base to hold it. I started with the drivers side and the movement was + - .002. I figured that with the quality rotors they were "true" out of the box. I put the other one on and bolted everything back up. Would I feel any pulsation if there was a problem? How can air get through the seals and brake fluid not get back out?
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
I figured that with the quality rotors they were "true" out of the box. I put the other one on and bolted everything back up. Would I feel any pulsation if there was a problem? How can air get through the seals and brake fluid not get back out?
The rotors should be true straight out of the box, however weather they spin true once bolted on is another matter. Which is why it's important to check the runout.
i could be wrong here but I dont believe you would feel any pulsation, as the rotor is still true, it's just not spinning true. You need to check the runout on both sides of the car.
the fluid most likely is escaping, but may not be easy to spot.
 

frank_nesta

Well-known member
Joined
Dec 24, 2004
Messages
199
Location
Los Angeles, CA
Corvette
1977 Convertible and 2017 Black Rose Grand Sport
Did you measure the lateral runout after installation? If it's over .008" or so, air can be drawn into the caliper past the seals. Try bleeding both calipers as a first step.

I agree. Try bleeding as first step. With new rotors, check the run out. Most likely sucking air, especially since you do not see any leaking.
 

Vettehead Mikey

Well-known member
Joined
Aug 26, 2003
Messages
3,486
Location
Not that far from Ottawa
Corvette
1973 Coupe
Well, sort of; like i said before, I purchased original GM brand rotors. I also purchased a run out gauge and a magnetic base to hold it. I started with the drivers side and the movement was + - .002. I figured that with the quality rotors they were "true" out of the box. I put the other one on and bolted everything back up. Would I feel any pulsation if there was a problem? How can air get through the seals and brake fluid not get back out?


It's not that the rotor is 'warped' out of the box, it's the fact that GM never intended owners to separate the rotor and hub/spindle in the field to replace one of the two components. They were machined by the factory as a matched set after riveting the two pieces together. It's not rare to see runout of .010" or more after a new 'flat' rotor is installed. Owners notice the pedal getting lower and lower as they drive as the caliper slowly fills with air from pad 'knock back'.

Try bleeding both calipers to see if the pedal feel returns.
 

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