Technical Brake problem

My friend is at his wits end with this problem. Bleeds brakes disc/ drum pumps up,gets a good pedal, a minute later loses pedal. No leaks or air trap in line. New master cylinder goes directly to proportioning valve, than residual valves 2lb f 10lb b to brakes. No metering valve in front. System worked prior to this problem. Maybe someone has some ideas I can relay to him

 

bad master

 

^ agreed. If the air is all bled and there are no fluid leaks its leaking internally past the seals.

 

2nd new master (corvette style)

 

If it is a ford explorer rear axel you have the calipers on the wrong side. The bleeders are in the wrong spot.

 

sorry forgot, disc on front. bleeders are up

 

both masters from the same manufacturer? Rebuilt or new?

 

Don't know if new or rebuilt. I think he deals with NAPA

 

You know,
if you can get one bad master in the box you sure as hell can get two. Seems to be pretty common anymore.

Plug the ports at the master and see.
That completely isolates it and its a pass or fail situation. Don't matter what or who says its new or good.

 

I'll tell him!

 

It was bench bled first?

 

If you have a good pedal when M/C is blocked off and pedal still goes to the floor, try pinching all the brake hoses at one time and try the pedal. if the pedal is high you have air in the calipers/wheel cylinders. Release one hose at a time to see when pedal falls. There is your problem. Good luck

 

Are the back brakes adjusted up?

 

If the master bypasses internally (no fluid loss), you lose the pedal, but the next pump of the pedal should bring it back up. Was the master the only thing that was replaced, prior to the brakes working?

 

Is the piston diameter of the new master new master large enough to fill brake cylinders?

Had this problem on a 48 ford with a really nice adapter kit that used a mustang two power
brake master cylinder.

Problem was the small bore of the mustang master is 15/16 trying to fill 1 1/8 - 1 1/4 original
48 ford brake cylinders.

Sooo replaced the mustang master with a larger bore F-350 truck master cylinder. Got good
pedal.

Note: Had to enlarge bolt holes on truck master to fit smaller mustang booster.

.

 

The OP states this is a Corvette style master, so the piston diameter should be fine, at least supplying enough fluid to the system. I can go along with the 2 crappy masters in a row theory.

 

Car sat for awhile, went to bring it out brakes failed. Put new MC was ok then failed again after awhile. Going to work on it today, see what happens?

 

Did you bench bleed the MC???

 

After reading your post again, That is exactly what is happening! As far as bench bleeding I don't know? but I think he would have done that

 

Just for giggles.. wait until it gets dark. Go under the car with just a flashlight and look at every connection for a leak. They show themselves much easier using this method.

 

Ok, MC is good. Bled the brakes front left caliber had bubbles, bled till clear. still had problem. pinched 0ff front hoses, good pedal. released right still ok . Released left hose lost pedal! WTH! No leaks

 

Go over that again, this time release the left first. See if same result of if the addition on the right causes a lost pedal. That will tell you if its the combo of both fronts or in fact the left side is problematic.

 

Is something causing the left caliper piston to back off?

 

sounds like a caliper piston seal dried out and is allowing it to suck air on the return , happens when pads get low or calipers are not used for long time and dry out ..