Getting Disc Brakes to Retract

[Jeff Balazs]

Hey Joe;

You could try and back off the pedal adjustment a tiny bit and see if that has any effect. This adjustment on my truck is very fussy and needs to be spot on. Not sure if the car adjuster is exactly the same but I needed to have mine within about a 1/6 of a turn to have good pedal but still release.

 

Fwiw:  I left the internals alone in my Grand Cherokee M/C. I had installed but later removed two Wilwood 2# residual valves on the front brake lines.  I could not get the calipers to release with them in place. I installed disc's all around. I replaced all the lines with 3/16" sized tubing and since I used the rear axle and brakes from a Grand Cherokee I figured a M/C from the same model would work best with what I have done. I also installed a adjustable proportioning valve so that I could dial in the braking at the rear. So far this all seems to work quite well.

 

 

Jeff

[Jim Yergin]

Joe,

If you wanted to check if it is the residual valve, I would think once the brakes were locked, you could crack the line from the brakes into the residual valve and see if they release.

Jim Yergin

[Joe Flanagan]

Thanks, Jim. I'll give that a try. Right now, the way I'm getting them unlocked is to open the bleeder valves at each caliper. 

Jeff, I have tried adjusting the pedal and it had no effect. Increasing the free play doesn't unlock the brakes. 

[james curl]

When I put disc on the front of my 48 P-15 I left the residual valve in the original master cylinder with a 10 pound residual valve to the rears and a 2 pound valve to the front.  Had the same problem, removed the residual from my original master cylinder and solved the problem.

[Jeff Balazs]

Joe;

Got ya. Was not sure if this would come into play on the car type adjuster and arrangement. On my truck with the mods I have made it certainly did.

I never could get it to work properly with the two external residual vales in place. After struggling with it for quite a while I determined that they were holding enough pressure to cause this to happen. So I removed them and it all works like I expected it would in the first place. This was sort of contrary to the research I had done. When I spoke with a couple of guy's I know who build cars for a living they both told me that they had the same experience with the external residual valves and had stopped using them altogether.

 

Not certain how much of this applies to what you are up against. My approach to doing this brake conversion was based on using an unmodified M/C that was selected to be as close a match to the braking system that I was installing. I didn't like the idea of tampering with the internals of a M/C... and reasoned that if I got the one I did which was made to operate the 4 wheel discs on a Grand Cherokee then it should work fairly well with what I was doing on my truck. Of course this is just one way to look at all this. Surely there is more than one way to build this.

 

Good luck. I hope you get it working the way you want it to.

Jeff

[Joe Flanagan]

Thanks, Jeff. As I understand it, with the stock MC located as it is lower than the calipers, I need the residual pressure. So I don't think going without them is an option for me. Others have this setup and it works reliably for them. I just need to figure out what's going on. Just a matter of time. 

[Don Coatney]

When I installed a dual chamber low mounted master cylinder I used the 2# and 10# external residual valves as recommended by the supplier. Bled the system and had good pedal. First drive around the block the brakes locked and would not release about the third time I used them. They did not lock tight so I was able to drive the short distance to home in first gear. My issue was the pedal free play adjustment. Like Jeff it took me a few tries to get it right but once I did all was and still is well. I do not know if my dual chamber cylinder has internal residual valves as I bought it as a unit with the bracket specifically for my application using discs in front and drums in the rear.

[Jeff Balazs]

Joe;

Can you tell I am a truck only guy? Had not considered the height differential.......on the truck the M/C is still slightly above the bleeders.

 

Jeff

[Joe Flanagan]

Went at this again this weekend and it seems all is well. I got a new 2 lb. residual valve and installed it. I made sure the relief port in the master cylinder was not obstructed. My neighbor helped me bleed the brakes. The fronts seem to drag a little bit more than they should, but I can roll the car without much trouble. Reset the pedal freeplay, too. Everything seems to work as it should. I am not going to mess with this any more until I get the car out on the road and see what it does. Next up is the window garnish. Then installing the rear window. Then I can put in the drive shaft and see if this thing will move on its own.

[Joe Flanagan]

Once again, this problem is back. The front brakes are dragging. I had the push rod adjusted so that there was 1/8 of play in the pedal. I increased the play to see what would happen and it made no difference. It appears that one of the cups on the master cylinder piston is blocking the relief port. When I insert a wire in there, it touches the cup. I assume this is not supposed to be the case. Also, when I depress the brake pedal by hand, no fluid flows up through the relief port, which it always did before and which I assume it's supposed to. If I insert the wire into the hole and THEN push the brake pedal by hand, fluid flows through the hole and the front wheels turn freely. I recently rebuilt the master cylinder. I know the possibilities are either that I used the wrong size rubber cup or that I put it in wrong. I will swear that neither is the case but I've been wrong before. I guess there's nothing for it but to disassemble the master cylinder once again. Any other ideas will be welcome.

[Dodgeb4ya]

I have seen many times the M/cyl rubber cup and piston do not return -move all the way back against the M/cly stop plate behind the rubber push rod boot.

You think you adjusted the push rod right and it seems ok then later the brakes are dragging again. I looked into this issue by testing for complete piston return with out the push rod boot installed so I could watch the piston movement. Sometimes it would sometimes not completely come back against the stop.

The master cylinder bore micro finish, M/cyl pston return spring strength and rubber cup type design ect affect quick and complete piston and cup return to the stop plate at the end of the master cylinder.

Also the pedal return spring must be installed to keep the brake pedal pulled back against the adjustable stop bolt.

 

None of this applies to non stock brake parts.

Bob

Edited August 9, 2014 by Dodgeb4ya

[Niel Hoback]

Looks like you get to take that MC apart again. With the pushrod backed away from the piston, something is keeping the piston from fully retracting Maybe a weak or broken return spring?  Piston is binding in the bore? 

[Joe Flanagan]

When you insert a wire into that relief port, it should pass all the way through into open space, correct? And also, when you press down on the brake pedal, you should get a flow of fluid up out of that port, right?

[Joe Flanagan]

I'll tell you what, I'm going to look into that weak/broken return spring possibility and hope for a miracle. How does the stop bolt on the pedal play into all of this?

[Don Coatney]

I'll tell you what, I'm going to look into that weak/broken return spring possibility and hope for a miracle. How does the stop bolt on the pedal play into all of this?

The stop bolt simply keeps the pedal from slamming the floor board when it returns to the fully up position.

[Joe Flanagan]

Right now I have about an inch of freeplay in the brake pedal, but the piston seems to be compressed tightly. If that's the case, then the rod is not in contact with the piston and therefore I can rule out any cause having to do with the pedal, push rod, etc. Does that make sense?

Can I also rule out bad front brake hoses? As I understand it, this could cause my calipers to lock up but the master cylinder piston would still move freely. Does that sound right? I will be taking apart the master cylinder tomorrow, but if I don't see anything obviously wrong, I'm really going to be stumped. I'm 99% sure that the new cup I put in had 1 1/8" stamped on it. I just can't figure out how the piston could be compressed and then not be able to retract. I do have a 2lb. residual valve to the front brakes installed (and the original residual valve removed), but from what I understand, that has to stay.

[DJ194950]

Crack the fittings from the MC to the residual valve slightly and check if the piston in the MC moves reward with a release of any pressure. Tighten fitting to prevent air re-entering. A change?= MC Problem. No change = front brake issue.

 

Ideas,

 

Doug

Edited August 8, 2014 by DJ194950

[Joe Flanagan]

Thanks Doug. Do you mean no change =mc problem and a change=brake problem?

[Joe Flanagan]

Never mind I just reread your post.

[DJ194950]

Sorry not the clearest post!

Front brakes are the issue I assume, not 100% sure, but you are doing the checking. With a single MC if could be front or back if lines/hoses/brake cyls. the problem area or still the MC.

Just 1 check. Hope to help get your project to a full time driver!

 

Doug

[Joe Flanagan]

Yes, Doug, it's just the front brakes. I've verified that the rears turn freely.

[DJ194950]

Sorry Joe to mention the rears again.

I DO remember earlier post and checks, but just in case of something floating in the system causing a problem, I brought it up again, Always re-confirm-even if it seems at pain- I know.

I do know that you are a very,very detailed person after all these posts and I only try to make thoughts that may make sense for you to look at.

 

Q.? You took apart the calipers and cleaned per an earlier post, but did you replace all the seals/boots etc.?.

The pistons should have micro-polished finish, unlike a brake cyl.  or MC. Fit is of major importance.

 

Again, just thoughts.

Have a good weekend, goodnight!

 

Doug

[soth122003]

Something to think about. If you haven/t replaced your front brake hoses, and they are swollen inside the pressure can cause the brakes to lock up. That same pressure may cause a slight vacuum on the line back to the MC and not let it return to it's proper position. I've seen 3 year old hoses that looked great, but were swollen closed inside.

[Joe Flanagan]

Doug, I think you're right that it's a good idea to confirm that the rear wheels turn freely. I'll check that today. As far as the calipers go, I replaced them a couple of weeks ago, so it's unlikely they're the problem.

I might also switch out the hoses today. If that doesn't fix the problem, I'll take apart the master cylinder again. This is the only thing that's keeping me from driving the car.

[Don Coatney]

Are you using a hydraulic brake light switch and if so where is it located in the system and do the brake lights stay on when the wheels lock?