Getting Disc Brakes to Retract

[Joe Flanagan]

Yes, I'm using a hydraulic switch. It's located on the frame near the rear axle. Yesterday after driving the car a bit, the brake lights were on and stayed on after I parked. That's the first time I noticed that. I dont know if it's been a regular thing.

[Don Coatney]

That being the case you might want to pull the rear drums even though it is a PITA and check the shoe return springs.

[Joe Flanagan]

OK. If the rear shoes were unable to retract would that cause the fronts to also lock up because the pressure is generalized throughout the system? And if I determine that the rears turn freely I can conclude that they aren't the problem, correct?

[Don Coatney]

If the rears are holding pressure in the system not allowing the MC piston to return to the neutral position the fronts will also not bleed back. What residual valve are you using on the rears?

[Joe Flanagan]

I'm using ten lb. Valve for the rear, two lb. for the front. I just checked the rears and the turn freely. They are a bit stiff but I assume that's drag in the rear axle and drive shaft. In any case, it doesn't take much to turn the rear wheels by hand whereas the fronts are locked up tight.

[Don Coatney]

Blue valve to the fronts, red to the rear?

 

[Joe Flanagan]

I will double check that.

[Joe Flanagan]

Yes. I have blue to the front and red to the rear.

[Joe Flanagan]

I just cracked the drivers side bleeder and that freed the wheel but the piston didn't really retract in the master cylinder, at least not that I could notice by looking at it.

[Jeff Balazs]

I should probably stay out of this......but I am very curious how one goes about getting a balanced braking system with this particular combination of modifications. I am really having trouble understanding how you would get an original type master cylinder to operate both types of brakes properly and how you would be able to adjust or balance the front to rear bias in the system?

Are you able to back the rear drum adjustment off enough to accommodate a front disc brake bias? It seems like a bit of a stretch to me that this would work consistently over time. Wouldn't you have a better chance of getting a more reliable and better balanced system if you used a M/C that was designed for front discs and rear drums?

 

Jeff

[Don Coatney]

I ran the original master cylinder with a drum/disc setup for many years without a proportioning valve with good results. My master failed a few years back and I replaced it with a dual chamber cylinder once again without a proportioning valve with the same good results. 

[Joe Flanagan]

I believe Tim Adams did this, too, as well as Norm (Norm's Coupe). 

[Joe Flanagan]

I just disassembled the master cylinder and I see nothing out of place. All the parts are in good condition. The cup is the correct size (1 1/8). A look inside the bore shows no damage or anything out of the ordinary. I also verified that the relief port was clear. My plan is to reassemble the cylinder, then operate the pedal while I watch the piston through the open filler hole to see if it moves easily back and forth. If there's a problem with binding, I would think it would show up during this test. There's enough brake fluid on the parts and in the cylinder to lubricate everything so I won't be trying to operate it dry. If all is well there, I'm going to fill the system, bleed it, and see what happens.

[Jeff Balazs]

to prevent the rears from locking up ahead of the fronts.

I ran the original master cylinder with a drum/disc setup for many years without a proportioning valve with good results. My master failed a few years back and I replaced it with a dual chamber cylinder once again without a proportioning valve with the same good results. 

Don;

So you were able to get a safe panic stop by simply adjusting the rear brake shoes?

I am curious about this because even though I used a closely matched M/C when I did my 4 wheel disc conversion I still needed to adjust the Wilwood proportioning valve to prevent the rears from locking up ahead of the front brakes.

 

Jeff

[Don Coatney]

I am running Dodge Charger self adjusting rear brakes. One time, first drive after the car had been sitting all winter, the rears locked on a hard stop. I believe there must have been a bit of rust on the drums that cleared up after that stop as I did a few test panic stops after that happened and the problem was gone.

 

[Joe Flanagan]

I reassembled the master cylinder and it moved freely when I operated the pedal by hand. I filled the system and then a neighbor and I bled the brakes. The first thing I noticed was that fluid was not rushing up into the reservoir when the pedal was depressed. Took it for a drive and the same thing: The brakes are locked up. 

 

I took a wire and pushed it into the relief port and I immediately hit what I'm sure is the rearmost rubber cup of the master cylinder assembly. When I did that, fluid rushed up into the reservoir. So the port is being blocked by that cup. The piston is out of position. Is that because of wrong parts or because it just won't retract? You got me. I think the parts are right. One thing I did notice was that I have a pretty tight bend in the line going to the front brakes. It's not a kink, but it's tight. Could this have an effect? The bend is right where the line comes out of the residual pressure valve on its way to the front brakes. 

Edited August 9, 2014 by Joe Flanagan

[DJ194950]

Try cracking the fitting  where the line comes out of the residual valve.. Let it bleed any pressure out- tighten.  Recheck if brakes locked after making them lock as before. If it frees front brakes and-

Check to see if the piston/cup in the MC are now clear of port. Change or No?

 

I'm starting to believe you have a return spring problem In the MC. Binding-too short- not enough spring pressure?

 

Doug

[greg g]

I would relieve that bend if you can. Remember you are pushing with a lot of force but return force is negligible, and restriction may be causing the problem.  It may be leaving residual pressure on the calipers. Causing heat and heat is no good if you are not in the process of stopping..

[Jeff Balazs]

Don;

I can see where this drum upgrade would help alleviate the condition.....but what happens when the stock rear brakes are retained? Perhaps balancing the braking bias is less of a issue with the cars than it is with a truck? I suppose the difference in weight distribution is significant enough.

 

I know I spent quite a bit of time getting the brakes just so on the truck. Last thing anyone would want is to do all this work and have something that tries to swap ends in a panic stop.

 

Jeff

[Joe Flanagan]

Doug: Thank you. I will try that tomorrow.

 

Greg: That's on my list, too. 

 

One thing I have not done is try new hoses on the front. I guess that's still a possibility.

[Dave72dt]

Go back to the master.  The cup must clear the relief hole with the piston fully against it's stop.  If it does not clear it, piston or cup or both are wrong.  Cracking your line between the hoses and the master and wheels free up eliminate the hoses and anything else between the calipers and where the line was cracked. 

[Dodgeb4ya]

I have posted a picture of master cylinder 1-1/8" rear cup being wrong-too wide and can block the relief port.

You cannot use a regular common wide wheel cylinder cup in a master cylinder as it will block the hole.

A proper narrower width cup must be used as what comes in a master Cylinder Kit.

NOTE: Chinese kits probably come with who knows what.

Bob

Edited August 10, 2014 by Dodgeb4ya

[Joe Flanagan]

Wow, that is really interesting. I wonder if that's my problem. I believe I got my kit from Roberts. 

[Adrian41D-19]

Doug: Thank you. I will try that tomorrow.

 

Greg: That's on my list, too. 

 

One thing I have not done is try new hoses on the front. I guess that's still a possibility.

i have the same trouble on my front brake on my 41 dodge, old brake hoses, original return spring, the front brake lock up, 

 

this is the color of the brake fluid when i buy the car ! first i belive this is an old motor oil ^^  when i opened the brake hoses i find a kind of soil or rubber ... by the way i clean all the brake line and replace some of them.

 

 

 

when i buy the car, i think all this parts are all original or very very old ha ha ha ^^ i replace every thing of the brake system i just keep the body of the MC and change the piston with a NOS rebuild kit

[Adrian41D-19]

I have posted a picture of master cylinder 1-1/8" rear cup being wrong-too wide and can block the relief port.

You cannot use a regular common wide wheel cylinder cup in a master cylinder as it will block the hole.

A proper narrower width cup must be used as what comes in a master Cylinder Kit.

NOTE: Chinese kits probably come with who knows what.

Bob

unbelivable ! ...    as much as possible i buy NOS parts !! and i know why now !!