I've read every relevant thread on the topic but am stuck at the point I'm at. Working on a 1975 automatic Vette. Power steering/brakes.

Back story:

- Got the car and been taking one item at a time to make her road worthy. Needed to rebuild the steering gear box so had to remove the master cylinder. Brakes worked fine up until now.

- Reinstalled gear box, emptied, cleaned, and bench bled the MC. Put back on car, and bled brakes. All were great except for the FR. This one kept putting out air, and a lot of air, not just a bubble here or there. Figured I'd worry about that later since I needed to get my carb and radiator fans in order. Got those in order through some other threads I had (thanks for the help!!).

- Figured with just the front right caliper having an issue, the car would stop fine. Not so. Pulled her out for the first time, and immediately realized there was almost no breaking power and a lot of travel from the break with no resistance until the very bottom.

- Backed her into the garage, up on stands, and notice a leak from the drivers side firewall area below the car. It was the proportional valve (as listed in another thread). Had a forum member here mail me the o-rings to rebuild the valve part of it, but also ordered the whole assembly online as I really needed this car for tomorrow and Sunday. Figured which ever got here first would go on the car.

- The whole valve assembly came first, put it on the car last night. No leaks, all looks good, bled the system, same issue with the FR with a lot of air, but now...no brake at all. The pedal goes right the floor.

- Read a ton of threads, figure the quickest and most likely culprit is the MC. Maybe it's gone bad. Purchase a new one today, bleed it, install it...same issue. Pedal to the floor.

- While bleeding I notice that when I pump, I get some resistance (while bleeding the rear). The resistance builds up, the releases when the fluid is released form the caliper while bleeding, pedal travels to the floor. Once we start bleeding the front right....no more resistance builds up, just the pedal to the floor. And that's where we sit now.

Note, there are no leaks anywhere no, pedal simply travels all the way to the floor. New MC. All lines are free of air except for the front right.

Only thought I have is to rebuild the old proporational valve with the o-rings I received, and reinstall it tonight. Maybe the new one is defective. But...I have two problems to fix.

1.) Pedal travel all the way to the floor.

2.) FR is full of air, but not leaking anywhere.

Sorry for the novel. Anybody have any thoughts or ideas where I should go to next with this??!?!?!? I'm completely lost. Not to mention, I'm no mechanic, I'm learning as I go with all of this.

 

Simplest, is to first find a plug for the front of the master and install it. Napa or other good parts store should have a short brake line with the right fitting you can solder/braze shut to use for a plug.
That will tell you if the front section of the master is good.
reconnect the front line with the RF bleeder open and let it gravity drip, then do the normal 2 person bleed if you don't have a pressure bleeder.

The front part of the proportioning valve is really just a tee if the spool is centered (brake warning lite off).
It doesn't take much air to floor the pedal.

 

I spent a bunch of years being frustrated with manual brake bleeding. I finally broke down and bought a pressure bleeder. Best purchase I ever made. It wasn't all that expensive and I've never had a problem bleeding brakes ever since. I highly recommend it.

 

I had the same problem, except it was the left rear. It's a bad RF Caliper. It's sucking air past the seals. Check it very closely for moisture (brake fluid) around the pistons. I'll bet if you change that caliper and do a rebleed it'll be cured.
Good luck,
John

 

Thanks for the tips folks. Keep em' coming. Going to try some things tomorrow morning.

 

Did you bench bleed the new master cylinder? Do you have the correct replacement master cylinder?

 

Easy Mike. Yes, both correct MC, and did bench bleed.

But...we have SUCCESS.

Per your folks' recommendation, removed the FR caliper. Although there was no visible leak, when we took it off, it was leaking all over the brake pads on the inside (could not see that leak without taking the caliper off). Easiest and quickest fix for me was to buy a replacement caliper and install it. Re-bled the brakes without issue...and now I have brakes!!

Thanks much for the help folks!!

 

must have had a cut piston seal (or a defective seal)....