Rear brakes locking up

[mjb765]

I have a 70 Cuda with manual disc/drum setup. There is a distribution block under the master cylinder and what looks like an aftermarket proportioning valve with a knob mounted over the rear axle before the "T" block on the rear end. The rear brakes are staying engaged and dragging. They were locked today and the only way to free them was to open one of the rear bleeder valves on a wheel cylinder. That part under the master--is it just a block or do they go bad? I can post a pic of the block under the master if needed if the opinion is that it may not be factory either.

 

[Cuda Hunter]

Yes, post a few pictures of the system.

 

[mjb765]

This is under the master cylinder---doing some research it looks factory. Do they go bad?? I'm sure the proportioning valve over the rear axle is aftermarket and at some point I would like to eliminate it. Maybe even a rear disc conversion and clean things up. I would even consider a complete system so I know everything is correct to work together.

 

[Challenger RTA]

I would say also check the residual valve in the master cylinder is stuck. Mostly because the is still pressure in the rear. It should keep 5-7 psi to the rear. Distribution block balances the psi front to rear. It could be that. This is a 71 setup.

 

[mjb765]

I just had the master cylinder rebuilt since I couldn't find a correct one......but I did notice that there is a delay in the brake lights going off after I release the pedal.....was thinking the master would be suspect.

 

[Challenger RTA]

Yes they go bad. Mark the valve. then open it the whole way. It shouldn't be there with a factory setup. Maybe it was used as a parking brake?

 

[mjb765]

Yes they go bad. Mark the valve. then open it the whole way. It shouldn't be there with a factory setup. Maybe it was used as a parking brake?

I think the front discs were added, but it is a OEM setup. I almost want to buy new aftermarket front and rear disc, new master and whatever else goes with the kit so I know it will work properly instead of trying to figure out what goes with what.....

so that block is not stock? it even has a spot for the sensor.

 

[moparleo]

Just an opinion. Unless you are road racing or live on top of a mountain, 4 wheel disc barkes are just eye candy and usually lead to pedal problems. The factory disc/drum setup is more than capable of stopping the car.
Stay away from rebuilt master cylinders. very poor track record.
Upgrade to power with a new mastercylinder/booster and upgrade the rotors and pad materials. Ceramic work very well.
Of course new bearings, races, seals, and brake hoses if more than 10 years old or showing signs of cracking, swelling.
First thing to do is a visual, rear brake inspection. Remove the drums and look inside. Many different things can cause rear wheel lock-up. Leaking axle seals, leaking wheel cylinders, stuck or broken brake hardware. plugged brake lines etc...
If it looks like rear brake time. Just buy top quality components and do it complete.
New drums, shoes, hardware, wheel cylinders, self adjusters, brake system flush. Bonded lining only, no riveted shoes.
Spend your disc money on things that you really need. I would replace all my brake lines and stick with factory style lines, avoid s/s.
The flares are very hard to seat and leaks are common with these.
Remember that there is also 1 rear flexible brake hose as well.

 

[mjb765]

Just an opinion. Unless you are road racing or live on top of a mountain, 4 wheel disc barkes are just eye candy and usually lead to pedal problems. The factory disc/drum setup is more than capable of stopping the car.
Stay away from rebuilt master cylinders. very poor track record.
Upgrade to power with a new mastercylinder/booster and upgrade the rotors and pad materials. Ceramic work very well.
Of course new bearings, races, seals, and brake hoses if more than 10 years old or showing signs of cracking, swelling.
First thing to do is a visual, rear brake inspection. Remove the drums and look inside. Many different things can cause rear wheel lock-up. Leaking axle seals, leaking wheel cylinders, stuck or broken brake hardware. plugged brake lines etc...
If it looks like rear brake time. Just buy top quality components and do it complete.
New drums, shoes, hardware, wheel cylinders, self adjusters, brake system flush. Bonded lining only, no riveted shoes.
Spend your disc money on things that you really need. I would replace all my brake lines and stick with factory style lines, avoid s/s.
The flares are very hard to seat and leaks are common with these.
Remember that there is also 1 rear flexible brake hose as well.

I appreciate the feedback and while those are all good ideas, the original problem was solved by releasing the fluid pressure at one of the rear cylinders. Then the question became is what I pictured stock or not. Instead of trying to troubleshoot a mix of parts, I was just thinking about upgrading the system front to back which is why I mentioned it. Doing what you suggest is not a bad idea, but the problem is not at the wheel itself, but in the fluid flow in the system. Releasing the pressure solved the issue, but of course it happened again. The front rotors don't have many miles on them and the pads were upgraded. This wasn't touched on, but the rear axle seals and bearings were done when the center section was swapped last fall. The master cylinder was done less than one year ago, so along with that came fresh fluid.

 

[Cuda Hunter]

With all of the replaced parts it sounds like your already narrowing down the issues. I would think one of the blocks are bad and just need a replacement. If the tail lights are staying on a little long that would probably be a spring and sensor issue that is under the dash.
I'm not expert or mechanic, so the guys that have mentioned stuff above have great information and have helped me with a few problems here and there over the years.

 

[Challenger RTA]

There might have been or still is more than one problem. Will it reoccur at the wrong time.

1 You said: "I just had the master cylinder rebuilt since I couldn't find a correct one." That's not a problem. Did you clear and check the residual valve?

2 "notice that there is a delay in the brake lights going off after I release the pedal" I would say that is the proportioning valve not resting for a number of reasons. 1 it's hanging up from wear or contamination. 2 Or the residual valve in the master cylinder is not working correctly.

3 the original problem was solved by releasing the fluid pressure at one of the rear cylinders. That resolved the current problem but what was the cause? Master cylinder,proportioning valve, Drums,wheel cylinder,rotor caliper,rubber hoses,hardware hanging up.

 

[Challenger RTA]

I was just thinking about upgrading the system front to back which is why I mentioned it

The best thing to do is put bigger rotors on the front,it does 60-70% of the braking. The next best thing would be rear ABS. I agree with Moparleo about rear disk brakes. Plus the bearing debate issue I'm not comfortable with. I know I have early 70s Imperial rear disc brakes. 2 sets of hen's teeth some nos parts if not 3.
I would have already put them on. The reason they where used they were on a 6000# land yacht.
The rear drum brakes when put on correctly are good for a long time.
Just give it a thought.

 

[Challenger RTA]

More on brake issues. This is to help others with problems.

The story goes. My son had a car problem used mine for bit. When he got it I told him I need to look at the brakes. I smoked the tires and brakes. Someone pulled out in front of me in a 45mph zone.
He brought it back and was starting to grind. He offered to help fix things. Not a big deal I'll get it later.
Later came put pads on pumped the peddle to take up for pad gap before it moved. Seem fine. Started the car and the peddle went right to the floor when running. Ok now what? Did not break loose any lines.
So by chance I had to meet someone to take possession and secure a property for someone. The current owner shows up and it's the mechanic that does my inspection. He deals with this stuff all the time. Day in and out for a living. I explained the problem. He said that happened once and a great while you need to bleed the brakes. But I didn't open anything.
Bleed the brakes. Went and bleed the brakes but only had a 1/4 of peddle.
I felt like there was air in the line. Bleed the brakes again and again. Also gravity bleed,bottle bleed, pressure bleed,vacuum bleed, bench bleed. Bleed lines from every connection. How many times I don't know. The brakes worked perfect until I put pads on the front. Ok had parts around that I knew were good. I know it's not this but I will do it to please the Gods of trial and error. Changed master cylinder around 4-5 times same thing. took master cylinders apart and check.Plugged the master cylinder checked for pressure and it held. Changed booster same thing right to the floor. Dissembled wheel cylinders and calipers cleaned and inspected. Same thing no brakes. At this point I know it's not contamination.Blew lines out and went though a 1/2 gallon or more of brake fluid. Checked movement of wheel cylinders and calipers. looked ok. I did get it fixed Not quite sure what it was but they work know! I'll show you next what fixed it I could not believe it.

I posted this here and added to it. Brake issues

 

[moparlee]

I think the front discs were added, but it is a OEM setup. I almost want to buy new aftermarket front and rear disc, new master and whatever else goes with the kit so I know it will work properly instead of trying to figure out what goes with what.....

so that block is not stock? it even has a spot for the sensor.

Don't think your question on the proportioning block below the master cylinder was answered. It does appear to be the correct block. It could be a factory one or a replacement, but in either case might need to be cleaned.

 

[mjb765]

Don't think your question on the proportioning block below the master cylinder was answered. It does appear to be the correct block. It could be a factory one or a replacement, but in either case might need to be cleaned.

Thank you...it looked factory and the research I have done on the part seems to confirm that. Based on the fluid release fixing the issue I am going to see if they are still locked in the garage from yesterday and i am going to try cracking one of the lines at the master open and see if that frees things up--if so then it might be the master cyl....if not will just work from there.

 

[Challenger RTA]

Awhile back I had problems with mine. This is a 71 build date 11-70. Don't know when or why they started to use the front metering block. Took it all apart thought I took pictures. Still Looking. Must not have or lost them for now.
The FWS cut away view explains a lot. Be sure to remove the sw first.

 

[Challenger RTA]

Wayne, I just redid my power disc/drum brakes on my 70 A66 Challenger. I have the two piece front to rear brake line with a proportioning valve on the frame underneath the drivers seat. First thought; get the proper Mopar brake master cylinder. I put the brake valves together outside the car copying the original valves I removed.(Pic 1)
Important note; early 70 Challengers had the two piece back to front brake lines, about mid-year the proportioning valve was integrated into the valves by the master and a one piece front to rear line was used.
Pic 2 proportioning valve on frame. The diagram shows where the individual lines go.
Let me know if you need additional information
Phil

View attachment 117370

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[mjb765]

Thank you!

 

[EV2RTSE]

I have a 70 Cuda with manual disc/drum setup. There is a distribution block under the master cylinder and what looks like an aftermarket proportioning valve with a knob mounted over the rear axle before the "T" block on the rear end. The rear brakes are staying engaged and dragging. They were locked today and the only way to free them was to open one of the rear bleeder valves on a wheel cylinder. That part under the master--is it just a block or do they go bad? I can post a pic of the block under the master if needed if the opinion is that it may not be factory either.

I parted a 70 318 Challenger with manual drums some years ago. If I recall correctly it had the same Weatherhead unit with warning switch, just as what you have pictured. There's not much to it, take a look at this article from musclecarresearch.com

Article-
1967 - 1975 Mopar Brake Tube Tee Autopsy | Muscle Car Research LLC
Parts-
Mopar Parts | Muscle Car Research LLC

The ones with the brake switch and proportioning valve are for cars with power discs, these had the running changes to them in the 1970 model year as pictured in this attachment. The later one was made by Kelsey-Hayes (KH).

So it would seem when they added the discs to your car they also added the proportioning valve at the rear to adjust front / rear braking bias. From what I understand, commonly done & nothing really wrong with it.

There were no manual e-body disc setups from the factory in 70 so I'm curious what master cylinder you had rebuilt - was it the original manual drum brake one?

Good article on factory master cylinders here-
Pumping Points

I'd also suggest checking for a possible internally collapsed or blocked rear brake hose, the one that connects to the rear axle.

 

[mjb765]

I parted a 70 318 Challenger with manual drums some years ago. If I recall correctly it had the same Weatherhead unit with warning switch, just as what you have pictured. There's not much to it, take a look at this article from musclecarresearch.com

Article-
1967 - 1975 Mopar Brake Tube Tee Autopsy | Muscle Car Research LLC
Parts-
Mopar Parts | Muscle Car Research LLC

The ones with the brake switch and proportioning valve are for cars with POWER discs, these had the running changes to them in the 1970 model year as pictured in this attachment. The later one was made by Kelsey-Hayes (KH).

So it would seem when they added the discs to your car they also added the proportioning valve at the rear to adjust front / rear braking bias. From what I understand, commonly done & nothing really wrong with it.

There were no manual e-body disc setups from the factory in 70 so I'm curious what master cylinder you had rebuilt - was it the original manual drum brake one?

Good article on factory master cylinders here-
Pumping Points

I'd also suggest checking for a possible internally collapsed rear brake hose, the one that connects to the rear axle.

I have the disc brake master cyl..the one on the right in this photo