Brake bleeding did not work

[Ted K.]

Did it all. Bled the brakes every way, gravity, wife, suction, and pressure, replaced master cylinder, and finally the rubber brake lines in the front, all with no change. The pedal firms up about one half inch from the floor, stops the car but you can't lock up the brakes, not enough pedal. The question is, is there an adjustment in the up and down movement of the pedal. I noticed a screw type adjustment on the end of the shaft that pushes the piston forward inside the master cylinder. That is the only thing that I can think of that might be left that I have not tried.
Is there any specification as to how that should be adjusted, if at all? It is power brake car.

[Bob H.]

Re: Brake bleeding did not work

question for others, could the booster not be depressing the m/c far enough? just remember having to adjust the rod length on a c2

[Edward J.]

Re: Brake bleeding did not work

Ted, I would not touch the booster adjustment if you did not replace booster, did you bleed the master cylinder before install? another thing I sometime will do when gravity bleeding the brakes; just unbolt the master cylinder and move away from the booster push rod and then do your thing, Its slower but works for me.

[Richard M.]

Re: Brake bleeding did not work

question for others, could the booster not be depressing the m/c far enough? just remember having to adjust the rod length on a c2

I have experienced similar issues. In one case, after using a rebuild kit on a original(sleeved) '67 MC, along with a replacement booster, there was a difference in the MC piston booster rod depth in the MC piston bore. The new Booster rod depressed the MC piston when the MC was installed and fastened to the booster.

I had to extend the MC mount by adding a thick gasket in between the MC/Booster mount flange. In my case the Booster rod was driving the MC piston too far.

Ted, I have a suspicion that the old MC was causing your overall issues, but the new MC may have a issue due to inaccuracies. When you installed the MC to the booster, did you notice if the booster rod was touching the internal bore of the MC piston or not? You may want to separate the MC from the booster and take some measurements of the MC piston bore depth and Booster rod for reference. When you install the MC to the booster, take note if you feel the Booster rod pushing the MC piston. It should not.

Rich
PS I have some measurements written down which I can find, but in the picture below I show the old and new MC pistons. The bore depths of the original and replacement were quite different.

[Ted K.]

Re: Brake bleeding did not work

I will remove the MC and make sure the booster push rod is not pushing the MC piston. I did see an adjustment screw at the end of the push rod when I installed the new MC but did not make any adjustments at the time. I assume it should be just touching the MC piston.

[Edward J.]

Re: Brake bleeding did not work

Ted, when you remove master to bench bleed , just take a measurement of the old plunger depth and the new.

[Ted K.]

Re: Brake bleeding did not work

I have removed the new master cylinder to measure the push rod from the brake booster to confirm that it just reaches the piston plunger in the MC. It appears to be correct, just barely touching. I will bench bleed the MC again and reinstall, and then pressure bleed all of the brakes again.
I have a feeling there will be no change. Are there any ideas left then have not been mentioned?

[David H.]

Re: Brake bleeding did not work

Ted,

Is it possible there is an issue with brass brake proportioning valve? (Not sure year in question.). Link below is example:

Dave

https://www.ecklerscorvette.com/corvette-brake-proportioning-valve-1974-1977.html

[Mark E.]

Re: Brake bleeding did not work

Did you bench bleed the mc per the manual?

Did you bleed all six caliper bleeder valves?

If yes, describe your bleeding process.

[Ted K.]

Re: Brake bleeding did not work

The Vette is 1973. I do not know what role the proportioning valve would play in low pedal. An explanation of ow it works would help.

[Ted K.]

Re: Brake bleeding did not work

All of the above was done properly with one exception. The inside bleeder valve on the left rear would not open. My assumption is that any air would have worked its way out by opening the outside bleeder valve on that caliper????

[Duke W.]

Re: Brake bleeding did not work

The Vette is 1973. I do not know what role the proportioning valve would play in low pedal. An explanation of ow it works would help.

The base disk brake system DOES NOT have a "proportioning valve". J-56 does, and it is adjustable.

The base disk brake junction block only has a differential pressure switch. Should one side develop a leak and not hold pressure when the service brakes are applied, the "Brake" light will remain illuminated even after the parking brake is released.

Proportioning valves are commonly used with disk/drum systems to limit rear line pressure. Due to the self-energizing (Duo-Servo) characteristics of drum brakes, rear line pressure must be limited to avoid premature rear lockup at or near the limit of adhesion. In GM-speak these are actually referred to as "combination valves" because they incorporate both rear line pressure limiting and the differential pressure switch.

The front rear brake torque distribution for the base disk brake system is about 65/35, which is computed by calculating front and rear piston area as a percent of total piston area. There is a simple formula to compute weight transfer during braking based on wheelbase and center of gravity. For the small block Corvette, which has about 50/50 static weight distribution, a 1g stop will yield about 65/35, but given that typical road tires will not generate this level of deceleration the fronts should always lock first, which is best. If the rears lock first the car is destabilized in yaw and can easily spin.

J-56 has the adjustable combination valve to account for different track surface friction characteristics and the significantly higher grip of racing tires.

Duke

[Ted K.]

Re: Brake bleeding did not work

The 1973 with power brakes does have a proportioning valve although not the adjustable type. I am assuming it would have no role in low brake pedal??

[Edward J.]

Re: Brake bleeding did not work

All of the above was done properly with one exception. The inside bleeder valve on the left rear would not open. My assumption is that any air would have worked its way out by opening the outside bleeder valve on that caliper????

Ted, you will need to bleed the air out of all bleeders, logic won't work as you might think.

[Bob H.]

Re: Brake bleeding did not work

Ted, that has to be the issue, air in that caliper. I believe you said you replaced the caliper and still had an issue?. Where is the bleeder valve on the inside, top or bottom?