My mechanic replaced my master brake cylinder last year. He used a cylinder that I had ordered from Eckler’s for a 1973 with power steering. When he installed it, he noted to me that the new cylinder was proportional -1/3 and 2/3 - and that the prior, leaking cylinder was 50/50. The calipers, rotors, and pads had previously been replaced and had very few miles on them.

I have since driven the car about 2,000 miles and noticed that the brake pedal was getting soft. Most recently, I began getting the red brake light on the dash appearing when I had to push the brake pedal pretty hard.

My mechanic is tied up for a couple weeks before he can look at it. He seems to feel it is again a master cylinder issue and I was wondering if anyone here has any guidance for us? Is the a proportional valve issue, perhaps? Just a need for a line bleed?

As always, I appreciate any thoughts from those of you who are a lot smarter on this stuff than me. The car will sit until I get this brake issue fixed.

Chuck in Toledo

 

Brake pedal slowly going soft and then the red light coming on generally means you have a leak somewhere.
But truthfully, without looking over the car we are all just guessing.
Look for signs of brake fluid on the inside sidewalls of the tires. Look for drops on the ground. It may at least point you in the right direction.

 

Not sure what your mechanic means by proportional Master unless he is referring to the size of the reservoir compared to a equal size rez 50-50
The piston diameter is of equal size, front / rear.

The Proportional Valve seldom needs replacement. Pretty basic, piece of brass, small piston / spring and a On / Off switch.
The warning light on the dash only comes on when the piston moves the plunger of the switch. And that can only happen from two known things: (+parking brake)
A loss of pressure in the system on any out-going port from the Prop Valve.
Or, a internal leak within the master cylinder. <<<These <<< are hard to detect because they are invisible. No drips externally, but leakage inside, past the MC piston seals.

Here's two things that are a must with buying C3 master cylinders:
The correct unit. Not a Chevelle, Camaro, GM, one size fits all MC, but rather a Corvette MC is a must.
The correct model year; GM changed design of the MC. More specifically the piston style at the mounting flange. Again, one size does not fit all.
As a side note, P.B. masters are different than std brakes masters as well..

And last but not least. Bench bleed before installing.
Click on my avatar, then check out my photo albums about the Prop Valve & bleeding the MC.

There is a test to check if the MC is holding pressure. It involves installing test lines.

 

This is why I appreciate this forum. I will provide this very helpful information to my mechanic. Thanks, HeadsU.P and 4-Vettes.

 

A soft petal could also be caused by air pumping into your system as a result of warped rotors. Is is not uncommon with rear brakes in later C-2 and C-3s. Jerry