Hello - In 2022, I did a full drum to disk conversion on all four wheels on my 1968 Camaro. This upgrade also included the replacement of the master cylinder, booster, proportioning valve and all the brake lines. The brakes work great for the first 12 months but then I started to notice that they would stick (momentarily) after I would brake to a stop and then begin moving again. For example, if I came to a stop on a slight hill, when I took my foot off the brake I noticed that the car would stay put. The brakes would retract after about 30 seconds (meaning the car would roll freely) so I assumed that I needed to do some general greasing of the slides on the calibers. Put the car up on jack stands and removed each caliber and greased the pins and while I had it apart, thought I would make sure each piston was moving freely. So as I had the caliper off, I would retract the piston to ensure it wasn't frozen. All the calipers were fine, ,meaning the piston was free to move, but after I completed the work on all of the wheels, I haven't been able to build pressure with the pedal to restore the brakes. I've bled them a number of times but since I can't get pedal pressure, all l will get is a squirt of fluid that is the equivalent to a pedal push. It's getting a little frustrating since the brakes were fine (minus the stickling) before hand and all I really did was push the pistons in to make sure they were operating properly.

My question is whether any of this sounds familiar to someone in the forum and/or do you have some thoughts on a way to isolate the problem so I can make progress. I am suspecting the master cylinder may be bad but it is just a hunch at this point. thanks for any feedback.

 

I suspect the MC. If there is a proportioning valve under MC, it may be stuck either on fronts or back or you need to use a tool to hold it while bleeding brakes

depending on what brand of brakes you have, the tool looks like this

. Classic Performance PV-TOOL Classic Performance Combination Brake Proportioning Valve Tools | Summit Racing

 

Is your brake lite hooked up? Is it on? I agree with SoCal but I would add have seen people push piston back so far in the caliper they seat and do not want allow fluid behind them. The quick fix to this is a pressure bleeder. This is the one case where a pressure bleeder is better than a vacuum bleeder or just letting the system gravity bleed. I remember a car like this way back in the day and the old timer next to me just said slam the brake pedal as hard as you can. It took 3 hits and it started working.

Is the conversion stock parts? Is the proportioning valve adjustable? Aftermarket parts like adjustable proportioning valves add in a lot of variables.

In my experience a mechanically sticking pad will not cause the issue your original issue. I have seen some pretty bad bound up pads and normally you just get a little more wear on bound pad. If you pump the brakes while driving with your hands off the wheel the car will lead towards the stuck pad. With rotors holding at a light is almost always a pressure issue, most likely a pressure not releasing issue.

If pressure bleeder does not sort things out and the light is working as it should (meaning the combination valve is not stuck to one side) I would get a pressure gage and hook that inline just to take the guess work out of the diagnoses.

 

Thanks for the response. I decided to replace the MC since it seems like the likely culprit. I also ordered valve tool you mentioned, I didn't know about it but sounds like a handy tool for bleeding.

 

Thanks for the note. I will try the master cylinder and go from there. Suspect it is the problem.

 

It is the square o ring in the caliper that does the returning of the piston
You hit the brakes, the pressure pushes the piston and distorts the square o ring into a trapezoid
After the pressure is removed the o ring goes back to square pulling the piston back
True, the master cylinder may be holding some residual pressure
You check the pushrod length, and it is not too long right?
If the new master cylinder does not fix it, plan on overhauling the calipers, using new o rings, dust boots and pistons too if needed

 

If the proportioning valve is sticking or partially stuck in either the front or back position...a new MC will not resolve the issue.

The tool link I posted is for bleeding when the PV needs to be held in the middle to allow bleeding. Those tools are specific to certain brands of PV as size and or threads is slightly different.

 

This is how we used to do it
You bleed one side of the system and then the other to center the piston in the proportioning valve
With a working brake warning light and the key on, you bleed one side watching for the light to go out
If it does not go out, you bleed the other side (front / rear)
Right when the light goes out you shut the bleed screw and put pressure on the brakes
That centers the valve
Some get stuck and cannot be re centered easily
Those get replaced
Good luck

 

Hello - Thought I would add an update to this issue in case someone in the future is experiencing a similar problem and searching the forum.

The master cylinder was the problem and replacing it resolved the no pressure problem. I also adjusted to pre-load when I installed the new one which solved the brake sticking problem I was experiencing. Thanks for all the comments and suggestions.

 

Wonderful
Going to do the same to my 68 soon
Where did you get the new master cylinder?