crause

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.

doorgunner

I don't have Power brakes.....but my project car did the same thing 24 hours after I rebuilt the entire system/new soft hoses...new m/c kit....new caliper kits...new pads.....etc.....the brakes felt great that day. The next afternoon----no leaks anywhere but the pedal went to the floor.

I ended up removing the rebuilt master cylinder thinking that I might have installed something backwards/everything was installed correctly.

I spent nearly 1/2 hour bench-bleeding the master cylinder (I DID NOT bench bleed it after rebuilding it originally to "save time"). It was difficult getting all the bubble out of the master cylinder, but after I re-installed it, it has worked perfectly for 15 months.

LB66383

If the seal has failed on the M/C, you should find brake fluid coming out of the back of the M/C. Loosen the mounting nuts, pull the M/C out a bit, and see if it is wet back there.

Did you check the runout on the new rotors when you installed them? Excessive runout can cause the caliper pistons to move around as you drive, and allow air to leak past the seals and into the calipers. Try bleeding the fronts again and see if you get any air out and the pedal goes back to normal.

Easy Mike

...but only after checking run out.

FirebladeDan

If it worked perfectly before I'd expect MC and connections to be good. I would ask a friend to gently press the brake while i go to every caliper with a flashlight and see if any of the caliper pistons are bubbling or making a noise. This would mean that a seal was unseated/cracked etc. LOOK VERY CAREFULLY FOR ANY BRAKE FLUID RESIDUE. Don't want to be replacing parts that are working.

If that doesn't work I would remove the MC lid and have a friend VERY GENTLY press the brake pedal. Make sure to wear glasses and cover the fender incase your friend has a lead foot. Stare into the MC and you should not see any bubbles. If it bubbles either your MC needs to be bled or bad seals...

If that doesn't work.. let me know i have some other ideas ha.

Sayfoo

I had similar problems a few years ago. I had a new/rebuilt mc that leaked out of the rear seal when bench bleeding. I returned it and got another one that would never work well--leaked internally. I returned it and they gave me my money back because, according to them, all the remanufactures were using cheap chinese internal rubber seals and they were getting a lot of returns. So, I bought a new, AC Delco replacement and it had a made in china sticker on it. It lasted about 1yr and 2k miles and then it would start to bleed down also. I now have a GM licensed reproduction that is supposedly cast in china, but machined and assembled with US parts. It has lasted for about 2 yrs now.
From my experience, it is probably the mc. But, I would bleed the brakes first, and check the run out, which is very important. If you have a lot of run out, it will always suck in air. Run out correction shims are available.

OMF

Back to the first post you say that the pedal was rock solid with the ports blocked on on the master cylinder..... this eliminates all this talk about booster rod length! The rod length is fine and the M/C is fine.....you have air in the system and the air is possibly stuck. If you have loops in the lines by the master cylinder, (not sure if that's done on a '69) air can get stuck in the top of the loops by having air in the top half of the line and bypassing brake fluid in the bottom half of the line ( yes all this in a 1/8" line!) If you have this scenario it will take a sharp hit of the brake peddle to move the air out of the loop, and make sure to have the bleeders open when doing this.
Raise the back end of the car to help the air bubbles move towards the calipers and watch for areas where bubbles a get trapped (another sharp hit may be required) and bleed it some more, inners first then outers...use a rubber mallet on the calipers and distribution valve.

Air is in there somewhere

HeadsU.P.

Check your dates.

OMF

OMG...FAIL