69 Charger , rear brakes locking up before front ones

[Bob T]

Hi Guys, after some advice please. On my friends 69 440 4speed, Disc front ,drum rear , boosted,  all stock ,  the rears lock up early and it needs to get sorted.
It'll be up on his hoist this week so I'm going over to give a hand. My guess is the prop valve is frozen and the plan is to get it out to free it. Is it a serviceable part? Or replacement only? 
What else should we be looking for to solve the problem, I've driven it previously, pulls up fine until recently.

Thanks

[Bob T]

Bump

[Paul G]

Does the pedal feel normal when applying the brakes? Or does it require a lot of pressure?

The proportioning valve can be the issue. I don't know of a way to test it other than to eliminate everything else. 

Crystalized front pads? Pads that no longer grip will cause you to apply more braking force until the rears stop the car.

Brake fluid leaking out of the wheel cylinder and getting on the rear shoes can cause wheel lock up.

[Bob T]

Ok, cheers Paul. The pedal seems fine for pressure feel, I'm leaning more towards prop valve at this stage though, just got to work through it and eliminate everything else.

[Lord Warlock]

More than likely one of the distribution block/proportioning valve or the hold off valve (the 2nd distribution block for the rear wheels has gone bad somehow).  the 2nd block is about a foot behind the main proportioning valve and was designed to delay the rear brakes from locking up before the discs in front did.  You can buy the hold off valve seperately from the main block if you need to replace them.  I just bought replacements for mine, haven't finished the front brakes yet or bled the lines yet so not sure how well they work but know you can get both if needed. 

[Bob T]

More than likely one of the distribution block/proportioning valve or the hold off valve (the 2nd distribution block for the rear wheels has gone bad somehow).  the 2nd block is about a foot behind the main proportioning valve and was designed to delay the rear brakes from locking up before the discs in front did.  You can buy the hold off valve seperately from the main block if you need to replace them.  I just bought replacements for mine, haven't finished the front brakes yet or bled the lines yet so not sure how well they work but know you can get both if needed. 

Thanks L.W. much appreciated, will look into that too 
The car is really low mile, it only has 38 thousand miles on it and everything is stock as a rock so it wouldn't have been touched before most likely.

[Lord Warlock]

after i posted last night, the thread disappeared and i couldn't find it again, I had another thread that showed the two distribution blocks in plain view so you could see what i was trying to point out, but then couldn't locate this thread again to paste it in.  If you search brake distribution blocks it should be easy enough to find.  But I do know the small one behind the big block was designed to delay the back brake a second or two to let the front brakes begin stopping.  The two block solution was supposedly only available on cars with power front discs and rear drum brakes.

If it were me, i'd remove the line that connects the two blocks together and blow it out with compressed air and see if there was a blockage, but the fact that the rears are stopping first makes me think the back is working fine, its the front that is the problem...which means it would be the main distribution block directly under the master cylinder that would be the problem.  May just need to bleed the front brakes or have a shop test them out.  I wouldn't think a 38k original car would have bad buildup, but who knows. 

[Bob T]

Ok, thanks for that L.W. much appreciated 

[GTXtreme]

does the master have a residual check valve in it?

[Bob T]

does the master have a residual check valve in it?

I don't know, I haven't seen it for a few weeks, I'll ask the owner tonight
Why, sticking perhaps??
It does have one of those big pointy noses and a towel rack out back but all the rest of the fitments are bone stock

[Dino]

Prop valves are known to lock up.  No harm in trying to clean them out but you may just want to bite the bullet and get a new one.  Towel rack huh?   

[Bob T]

Prop valves are known to lock up.  No harm in trying to clean them out but you may just want to bite the bullet and get a new one.  Towel rack huh?   

It gets them dry real fast Dino 

[resq302]

Almost seems as if the valve in the front to rear line might not be allowing the fluid to delay to the rear brakes.  I had that problem when my hold off valve had problems and I put a pig tail jumper tube in there just to be able to move the car around.  What that valve does is delay the rear brakes a split second so the fronts could engage first and prevent the back brakes from locking up and swinging the back end of the car around.

[69white hat]

I also have this problem on my 69 charger. The brake pedal feels normal but very touchy. I have to be careful or the rear wheel lock up or i get thrown to the steering wheel. Wife hates it. But other times the pedal goes right to the floor and I can barely stop. The rear brakes aren't working at all when this happens and I'm relying on the front brakes only at this point. Defintily something funny going on.  Getting it checked out next month when I take it to my mechanic.

[Back N Black]

I also have this problem on my 69 charger. The brake pedal feels normal but very touchy. I have to be careful or the rear wheel lock up or i get thrown to the steering wheel. Wife hates it. But other times the pedal goes right to the floor and I can barely stop. The rear brakes aren't working at all when this happens and I'm relying on the front brakes only at this point. Defiantly something funny going on.  Getting it checked out next month when I take it to my mechanic.

Have you checked you brake fluid level? sounds like you have a leaking wheel cylinder and fluid on the brake shoes causing the brakes to grab.

[NMike]

before you start replacing parts, have you actually checked the front calipers? after almost 50 years, i would be looking at seized pistons or sliders. pull the front wheels and see how hard the pistons are to push back. then have someone pump the brakes are see what they are like to go back out.

when all this is done, FLUSH the brake lines with good DOT4.

[1974dodgecharger]

before you start replacing parts, have you actually checked the front calipers? after almost 50 years, i would be looking at seized pistons or sliders. pull the front wheels and see how hard the pistons are to push back. then have someone pump the brakes are see what they are like to go back out.

when all this is done, FLUSH the brake lines with good DOT4.

That's what I did mine would lock up in front and would not release so she would be burning the pad.  Took her apart and regreased the insides etc.. She worked fine....

[Bob T]

before you start replacing parts, have you actually checked the front calipers? after almost 50 years, i would be looking at seized pistons or sliders. pull the front wheels and see how hard the pistons are to push back. then have someone pump the brakes are see what they are like to go back out.

when all this is done, FLUSH the brake lines with good DOT4.

Good point, thanks, its easy to overlook the obvious and think it is a totally different cause and effect.
I've been tied up with a new contract and haven't  been over to start checking it out. I've sent the link to the owner though, likely he will have a look shortly.
Thanks for the replies and tips Guys

[kenny]

What happened here??
I also have this problem,and i am running out of ideas.... 

[Bob T]

What happened here??
I also have this problem,and i am running out of ideas.... 

It turned out to be a failing spring which was causing the shoe to drag and not return, quite a simple solution really after all the theorizing 

Do appreciate the tips though, thanks guys