Thread: front calipers wont retract

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Guys I am stumped an need some help. I have a 39 coupe, GM front calipers, GM rear drum. M/C under the floor and 2# front residual valve and 10# rear residual valve. Everything in the system is new and brakes are bled.

When I apply the brakes, everything works front and rear. When I left off on the brakes, the front calipers wont retract but the rears do. What could be wrong? I know my lines are correct from my MC and the plunger rod comes all the way back after the pedal is released. The front calipers are around the same level as the MC. I am wondering if the front 2# valve is the problem. Maybe it wont let the pressure come back?

Sometimes the master cylinder will have a built in residual check valve, and its not really apparent. Try what Denny says, and if they back off, that may be the problem.

did you check the MC push rod .sounds like it is to long take it out and see if that helps

I think the residual valve may be your problem, and I think we have the same problem with my Son's T. He has GM discs on the front and they seem to be dragging all the time. You can turn the wheel, but it takes some effort. We have a 2# valve in line, and maybe the one in the M/C is doubling up on the pressure to the caliper.

I think I'll take the guts out of the residual valve and see how that works. Thanks for giving me this idea.

Don

Don--I can not remember where I seen a post about this, but I remember a photo looking right into the threaded hole in the master cylinder where the brake line usually goes into, and its a little brass thingie pressed down into the bottom of the threaded hole.---Brian

i have used the residual on some builds with gm masters and the chrysler masters and a ford they all work fine but i was told the the drum master did not check back any fluid??? and a disc front MC did you could try it with out any residual in the front but it may not work very good .i still would make sure you have free play in the pedal when released have seen this happen

Originally Posted by DennyW

Yeah Pat. That's why I suggested to start at the bleeder, and just do the very basic check first. This would start the process of elimination. Open bleeder, free spin...not the caliper. Open above the flex hose....free spin, not the hose....and so on, until he reached the master. And with the lid off, depress brake pedal (SLOWLY, as not to splash fluid out), and then release, and see if you can see fluid return to the master cylinder.

yep that will help.he need to start some were

from that description, I make the following assertion:

start at the master cylinder. See if fluid returns.

I popped the bleeder and the calipers retracted. I let the master cylinder push rod loose and it was adjusted full retraction so I assumed that the internal bypass valve was exposed.

I then took my front 2# residual pressure valve out and since I didn't have enough line to make a new one without the valve in it. I took the spring and rubber diaphram out and re-installed the valve which is basically open now.

My brakes work but I am concerned about not running the front pressure valve. I couldn't find anything that says the corvette master cylinder has an internal valve built in.

If your running front disc/rear drum you probably don't want to use a corvette master ---Your best choice would be a master from an late 60/early seventy Ford product that was for front disc/rear drums--they probably will already have the residual valves built in and your only concern would be matching front brakes to the front fitting and rears to rear

I have seen your complain on a few Cobras with mismatched components--there is a little difference on the spread of the mounting holes but a little time with a rat tail file will fix that

Jerry

Originally Posted by mrrilla

I popped the bleeder and the calipers retracted. I let the master cylinder push rod loose and it was adjusted full retraction so I assumed that the internal bypass valve was exposed.

I then took my front 2# residual pressure valve out and since I didn't have enough line to make a new one without the valve in it. I took the spring and rubber diaphram out and re-installed the valve which is basically open now.

My brakes work but I am concerned about not running the front pressure valve. I couldn't find anything that says the corvette master cylinder has an internal valve built in.

Bingo. I'm going to do the same thing and see how that works.

Don

If you have a master cylinder with built in residual pressure valves you MUST the the front/rear lines going to the correct fitting on the master---Also if the master uses a separate metering/proport valve you will need to use the proper one for your systems specs.
And is your master cylinder clean???
And what kind of fluid are you using? some seals are not compatable with some fluids

Originally Posted by mrrilla

I popped the bleeder and the calipers retracted. I let the master cylinder push rod loose and it was adjusted full retraction so I assumed that the internal bypass valve was exposed.

I then took my front 2# residual pressure valve out and since I didn't have enough line to make a new one without the valve in it. I took the spring and rubber diaphram out and re-installed the valve which is basically open now.

My brakes work but I am concerned about not running the front pressure valve. I couldn't find anything that says the corvette master cylinder has an internal valve built in.

Mrrilla
Is this part of a front disc brake/power booster conversion Kit? This is almost the exact same set up as I got from Classic Performance Parts. They have a tech line that is great for figuring out these problems.

This is not a conversion kit. It was built from scratch for a 39 chevy.The MC is an aftermarket "street rod style" and the booster is a standard 7". I think I had the front and rear lines crossed from the MC to the proporting valve so I changed that and nothing different happend. The proportioning valve has a built in metering valve and I am 100% the plumbing is correct. I am using a DOT 3 fluid in the system.

Everything is functioning now that I got rid of the front 2# residual valve. I still have the rear 10# in there. The system works great great under no vacuum but once I start the car, I have a long pedal throw. I think that is a simple pushrod adjustment or some air in the lines though.

Is it possible that the front residual valve was installed backwards and acting as a check valve?????