I am now on my 4th bad master cylinder in 4 years. The one I have now was a brand new DR. Both front and back fluid full. The car does sit a few weeks at time between driving so.

Is there a brand or retailer people have had good luck with?

 

A master cylinder should only need replacing if it is leaking. Is that what happens to yours?

If not it sounds like you have problems further down the brake system like leaking calipers.

 

What's missing here is the problem you are facing.

The Master Cylinder has 1 simple function, translate pedal movement into fluid movement. So I believe a simple test of pushing on the brake pedal and fluid bleeding out of any front caliper bleed hole and the same for any of the bleed holes in the rear would indicate the MC is working in the most basic function.

Any symptom like I can push the pedal to the floor , no brakes , brake light on could be a problem somewhere else.

So lets here what your brake problems are.

 

The brake pedal gradually fades all the way to the floor and the brake light comes on. There are no leaks in the calipers and master is full of fluid.

 

Air in the system.

 

Have you had a professional pressure bleed the system!

I am am as good of a Corvette mechanic as you can find who doesn't have his own shop and I know all of the tricks to get it bled, but I prefer to get it good enough to drive it down and give this job to a guy with brake bleeding equipment!

Also you aren't doing something funky like living at high altitude and using silicone fluids are you?

 

If you have excessive rotor runout, it is possible to be forcing some air past the caliper piston seals because of that constant vibration/movement. Check runout on rotors and compare with GM specs.

 

What you are referring to is knock back. When your rotor is running out it pushes the caliper piston back in. Since disk brakes work by having the pads in constant contact with the rotor, if they get pushed in (knocked back) the first time you hit the brakes you get little to no braking. Pumping the brakes will get you stopped but it is not a fun experience.

Run out does not cause air to get into the system, it simply pushes the piston in causing the lack of contact, which then has to be made up with extra pushes of the peddle. There are only two ways to get air in the system. Sucking air in from an empty master cylinder or boiling your fluid. You can only boil your fluid if it gets too much water in it, unless you are racing....like at a track.

PS you can get Knock back even if your rotors run true, if your wheel bearings are way too lose. When the wheel bearings are lose the rotor ***** between the caliper and pushes the pads away.

 

Soooo...you know what WON'T put air into the system....but you don't know what WILL put air into the system. Interesting....

 

Could be the MC piston seals are shot. If you want to test the MC you can hook up two dead-end short pieces of steel lines in place of the brakelines. NAPA sells pre-made lines. (Front and rear brass fittings are not the same size. If you need the dia & TPI of the brass fittings let me know). This test, using the brake pedal, will determine if the MC is bad. Unfortunately, you will introduce air when disconnecting the brakelines, so a complete bleed is in order, sorry.

 

I'd bet a few dollars that all of your previous master cylinders were functioning properly, and that your real issue is air in the system from rear rotor runout. The knock-back comment is interesting, but my experience with these cars says that runout at the rear rotors often does allow air into the system very gradually, causing a fading pedal over time.

 

When I first got my 69, 5 years ago it had a low brake pedal and gradually went to the floor. Replaced the master, then the pedal kept getting lower and lower. Rebleed brakes and within weeks the same thing happened. Come winter the car got the caliphers rebuilt an figured it should get the new rear brake lines. When installing the right rear I found where at the last time that line was replaced it was cross threaded into the brass elbow, hence sucking air into the system. These type of leaks could be anywhere in the brake circuit. I was lucky to find mine, but if you have access to a bleeder ball and can let it sit overnight , I'm sure the leak will show itself. T