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Rear brakes locking

Ricks72Chlgr440

Ricks72Chlgr440

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I have a 72 Challenger I purchased recently with stock style front disc and rear drum. When I hit the brakes hard, both rear brakes lock up before the front ones appear to be applying. Since the front brakes are supposed to provide the majority of braking power; any ideas why this would be happening?
 
Chryco Psycho said:
Missing / wrong / faulty proportioing valve , you may need an adjustable one to balance it properly .
Click to expand...
I found the distribution block on the car was for drum brakes, so I replaced it with the disc/drum one. Same result. I would consider an adjustable prop valve but am squeamish about cutting/running new lines for it. Would a different master cylinder be better? Not sure what is currently on the car.
 
moparleo said:
Have you pulled the rear drums and inspected the brakes yet ? Could have fluid/oil on lining, mis adjusted rear shoes, broken /faulty brake parts ...
Click to expand...
Yes I have, and are all in good shape. In fact, when I stomp on the brakes both rear wheels lock up together perfectly.
 
Another consideration is the brake warning light. When I press on the brakes gently, the light does not come on. When I stomp hard, the brake light comes on until I let up on the pedal. Thinking the MC may be leaking internally.
 
Leaking lines or components can mimic master cylinder problems. Check the fluid level in the master cylinder first when any brake issues develop. If the reservoir is full, you're not experiencing an external leak. If your foot slowly sinks to the floor with normal pressure on the brake pedal, the master cylinder is leaking internally.
 
Nothing hard about installing a after market proportioning valve in the rear brake line. Having the rear brakes lock before the front always means the rear end is going to swing around! Not fun in heavy traffic!
 
Challenger RTA said:
Ok need a picture of proportion valve meter block. let see what you have. Did you reset you brakes before bleeding? https://www.forebodiesonly.com/forum/attachments/brake-valve-jpg.82222/
Click to expand...
20231018_131728[1].jpg
 
Challenger RTA said:
Leaking lines or components can mimic master cylinder problems. Check the fluid level in the master cylinder first when any brake issues develop. If the reservoir is full, you're not experiencing an external leak. If your foot slowly sinks to the floor with normal pressure on the brake pedal, the master cylinder is leaking internally.
Click to expand...
No leaks & the master is full. Have not noticed the brake pedal sinking at all.
 
72 I'm going say you don't have a front block with the reset. as in the picture I posted. 73 was a different proportioning valve as I remember. The master cylinder has to be the correct one too. The master cylinder has a residual valve that keep 5 to 7 psi in the rear system. if it is not working correctly the brakes will lock up. I am not saying that the proportioning valve is good or bad. It can be taken apart carefully and cleaned and inspected. If there ever was water in the system even a drop. the valve will not work right. Dot 3 will attracts moister . It might be just stuck! The are a few post on brake problems here. If I find them I will link it. Or others may jump in.
 
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Challenger RTA said:
72 I'm going say you don't have a front block with the rest. as in the picture I posted. 73 was a different proportioning valve as I remember. The master cylinder has to be the correct one too. The master cylinder has a residual valve that keep 5 to 7 psi in the rear system. if it is not working correctly the brakes will lock up. I am not saying that the proportioning valve is good or bad. It can be taken apart carefully and cleaned and inspected. If there ever was water in the system even a drop. the valve will not work right. Dot 3 will attract moister . It might be just stuck! The are a few post on brake problems here. If I find them I will link it. Or others may jump in.
Click to expand...
Thanks. The original distribution block on the car and it was for a drum brake car, so thinking maybe the car came with drums. Dunno. I got this one just this week new from Dr Diff. I'm using DOT4 fluid also.
 
Being a new proportional valve I wouldn't think that would be the problem. but you never know. I would look at the master cylinder residual valves or pinched line.
 
Challenger RTA said:
Being a new proportional valve I wouldn't think that would be the problem. but you never know. I would look at the master cylinder residual valves or pinched line.
Click to expand...
I ordered a different master cylinder. No idea what's on there now so now I'll know. Will report back after the changes are made.
 
Before you put the new master cylinder on. Try jacking the rear or front of the car up to get the master cylinder level. The mc is mounted at an angle. so air will be trapped at the front. Have someone push on the brake easy, to see if there are any bubbles. this might be the problem air trapped. That's why it is suggested to bench bleed the mc.
 
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