Brake trouble

[Fern]

So I've been having this problem for as long as I've had the car. My rear brakes lock up easily. When you push down on pedal it goes about 1/2 way and catches all at once. Front brakes work fine ( I think) but back will lock up. Someone install power brakes before me and I'm not sure of what was used. I think it's the brake booster that's faulty but not sure which year to get 70 and 71-74 are different.

[ChallengerHK]

Someone will chime in here shortly, but I want to say that the actuator rod into the master cylinder has to be shorter for the power setup.

[4 speed fish]

Are you running the correct Proportioning valve?Sometimes when people put power brakes on their car they use parts that will not work together.

[Chryco Psycho]

The pedal travel issue could be a couple of issues where the shoe adjustment is off so the shoes have to travel too far to contact the drum causing them to grab instead of slide on the drum surface , it could also be the pushrod length causing excessive travel before the master cylinder us depressed to apply fluid pressure to the shoes  to contact the drum .
 The lock up is either the oil contamination on the shoes causing them to grab rather than slide on the drum surface , or the proportioning valve is not working correctly balancing pressure to the front & rear correctly or more likely the car came with 4 wheel drums & has been converted to front disc brakes but still has the 4 wheel drum proportioning valve allowing too much pressure to the rear brakes , this can be solved either by using an adjustable proportioning valve , the correct drum / disc proportioning valve or using different wheels cylinders to reduce the pressure for the drums

[Fern]

I changed out the proportioning valve and master cylinder.
All brake lines are new. All brake pads are new .
the only thing left is the booster.

[Fern]

How do I know if the rod length is wrong?
What length should it be?

Thanks

[ChallengerHK]

How do I know if the rod length is wrong?

I may have been wrong about my diagnosis. The more I think abut it, I think that issue causes the opposite problem (fronts lock up). List to Neil (Chryco); he knows what he's talking about. 

[Fern]

The pedal travel issue could be a couple of issues where the shoe adjustment is off so the shoes have to travel too far to contact the drum causing them to grab instead of slide on the drum surface , it could also be the pushrod length causing excessive travel before the master cylinder us depressed to apply fluid pressure to the shoes  to contact the drum .
 The lock up is either the oil contamination on the shoes causing them to grab rather than slide on the drum surface , or the proportioning valve is not working correctly balancing pressure to the front & rear correctly or more likely the car came with 4 wheel drums & has been converted to front disc brakes but still has the 4 wheel drum proportioning valve allowing too much pressure to the rear brakes , this can be solved either by using an adjustable proportioning valve , the correct drum / disc proportioning valve or using different wheels cylinders to reduce the pressure for the drums

There is no leaking on the brake pads that I can see.
I have the correct proportioning valve.
I adjusted the back brakes.
I do believe the car came with 4 wheel drums, but cant be positive.
You don't mention the brake booster at all, so would using the incorrect booster not cause this?
All I have left is the booster and pedals.

Thank you

[dutch]

Id`d put it on a brake tester first, so you will KNOW what is happening while braking.  I used an adjustable prop valve to tame the rear brakes.  They were really over reacting and it felt like all brakes kicked in at once, but the tester showed that wasn`t the case. Mine had pb, 11"drum in the rear and discs front.
Hotrod mag. road test in 1970 already talked about the same problem,  but back then there was still space on the roads to do a 180 or 360 at the light...

[spamtank]

There's a primary and secondary shoe.  Any chance they got reversed?  Could that cause the rear's to lock up?

[brads70]

When I got my car and had drums out back I used   (Raybestos WC37236, DORMAN  W37236).  The stock bore is 15/16" and the trucks is 7/8"

[72rtchallenger]

I agree with CP on what he posted , now you say it was converted before you owned it , did they swap to factory style proportional valve and master cylinder ? Some new aftermarket  proportional valves are adjustable . And make sure they got the brake lines routed and plumbed to proper connections , as the disk brake takes alot more pressure to work than a drum set up , could they have plumbed it backwards ? As unlikely as that may seem you dont know who did work before you 

[jhaag]

Fern, to answer your concern about the booster, it is unlikely that it is the cause. All a booster does is help assist with the mechanical advantage, to lessen pedal effort, needed to apply the brakes. I think you have a different issue causing your problem as pointed out in other posts. Unless you are married to the stock components, I would suggest eliminating the stock prop valve, and use an aftermarket adjustable one. This is of course is assuming you have eliminated all the possible issues pointed out by Chryco. 

[Fern]

I checked out the brake shoes and they are correct.
After reading what Chryco said and others (thank you ) I believe there's 2 problems here. I can change out the proportioning valve to a adjustable one. But I also think the travel of the pedal Is off. The pedal goes 1/2 way down and grabs all at once. I checked at the pedal on the interior and it sits right against the booster with no play. So if the rod length is off wouldn't it be in the booster?
Or am I missing something?

[wantone]

There's a primary and secondary shoe.  Any chance they got reversed?  Could that cause the rear's to lock up?

Any chance the star wheel adjusters are locked up too far?