Rear brakes lock up... front disc

-
I'll try to completely disconnect the ebrake cables and see what happens. As stated above I already have gone to smaller slave cylinders, "7/8" I believe on my 7-1/4 rear-end. They did help but did not eliminate the problem. BTW, I am using organic pads on the front. I'm starting to think they're just not grabbing properly.
 
Organics don't have the friction qualities of semi-metallics. Your fronts are greatly under braking even w 4 piston calipers . I run semi's on everything. The downside is you change rotors w pads everytime.
Make sure you manually retract the cable in its sheath. Basically, yank the shoe backwards away from the backing plate. That should fully slack the e-brake cables. The other fella was right also. Smaller cylinders make shoes spread quicker but require significantly more effort. No prob for power brakes but a real challenge for us manual folks. Good luck n be careful.
 
Jim, I'm having the same issues with my '73 Dart.
Front discs (11") and rear axle upgraded from the wimpy 7-1/4 to a 8-3/4" with 10" drums.
I don't know if the 8-3/4 has larger brake cylinders or not, but the rears lock up 'fairly easy' with my car aswell. Stock prop.valve.

I describe it pretty much the same as you do, it feels like the front brakes just aren't grabbing hard enough.

I've got an adj. propvalve, ready to try out, but since the front brakes need replacing (calipers/brakeshoes/rotors), I'm currently looking into upgrading to a larger discs and maybe Wilwood calipers to save some weight from the front.
 
My original brake set up used to always lock the rears up easily; factory manual disc. It was like power rear brakes. '69 Factory Service Manual says I should have a factory proportioning valve on the frame rail back by firewall under the floor board; my car never had that. I drove with these death trap brakes for many years. This rear wheel lock up issue is common with these cars. That is how these cars were built originally.

Now I run the 7/8" rear wheel cylinders (order for rear of '72 D100 2WD truck), MP adjustable proportioning valve, stock combination valve, 15/16" master cylinder (Order for '73 Duster 340 w/power brakes RAYBESTOS Part # MC36406 its a direct bolt on), Goodrich braided brake hoses front and rear, all new metal brake lines, new springs and shoes in rear, reproduction rear drums, emergency brake cables freed up and lubed, and front calipers not sticky and working correctly. Front brake pads are NOS. The car brakes fairly well, rears don't lock up. But when the front pads heat up it takes more pedal effort to slow the car down. In the summer I am not happy with the braking performance at all but rears don't lock up, just too much pedal effort when pads get hot. I just ordered EBC Red pads for the front. Rotors are in great shape so I'll just scuff em up with sand paper when installing the new pads.

I think the combo I have works well and I think with better pads up front I am hoping to finally have this crappy brake set up licked.
 
And yes, the smaller rear wheel cylinders apply less pressure to the rear shoes. This is the first thing that should be done on these cars. And its what Mopar should have done but they wanted to use the same wheel cylinders on everything back then so....
 
Rat Patrol said:
I don't see this as a "problem" - I see it as a known issue with all disk/drum Mopars.

I'm willing to bet that if every one of the members posting on this thread running disk / drum went out and did one emergency stop from cold - 98% would end up sideways.

Give it a whirl folks - you'll be horrified. :)
Click to expand...

It's a PROBLEM!

Regardless of whether or not every Mopar that left the factory does it or not, its still a problem!

And I can tell you right now that if I went out and did an emergency stop from cold that I WOULD NOT end up sideways. I know this because I've done it, ended up sideways, and went back and fixed the problem.

Most of us spend a lot more time figuring out engine upgrades, heck, even transmission upgrades, than brakes. But your brakes are the most important part on the car.

If your car doesn't stop right during an emergency stop- FIX IT.

Smaller rear wheel cylinders will help. Making sure you're running the same material on all your brakes (semi-metallic is best) is important too. And if that still doesn't fix it, then add that proportioning valve. Assuming, of course, that all the brake hardware is in good working order.

As I mentioned already, I run the later Mopar 11.75" front disks and 11x2.5" rear drums on my Challenger, and will on all of my A-bodies as soon as they're on the street (already have all the parts!). The braking force seems to be much better balanced "out of the box" than the 10.98" rotors and 10x2.5" drums. I still run the larger 15/16ths rear wheel cylinders, but with a 15/16th's master cylinder, full stainless lines, and all new hardware, pads and shoes. I also run drilled and slotted front disks. Tires make a big difference too, if you're running a staggered set up that can make a difference, as well as tire choice. The 275/40/17's I run all the way around make my brakes work for their money.

Regardless of your set up, spend the time to get it dialed in right. Brakes are one of those things that should only be stock on a museum piece. If you drive it, you should be able to stop it.
 
Sound like the shoes are to loose in the drum. Thay may need to be ajusted to were the shoes are barely touching. The other the thing the master should have 1/2 of play before it starts to brake.
 
I had the same problem. I put in new shoes and springs. Pads and shoes are Firm Feel's metal ceramic. Even with bigger tires they would still lock up. I added a proportioning valve from SS Brakes and dialed it down a half a turn at a time until they stopped locking (3.5 turns total). I spent two days at Watkins Glen and other than over heating them (all four brakes were radiating heat like a blast furnace) they worked fine, never locking up.
 
72bluNblu said:
It's a PROBLEM!

Regardless of whether or not every Mopar that left the factory does it or not, its still a problem!

And I can tell you right now that if I went out and did an emergency stop from cold that I WOULD NOT end up sideways. I know this because I've done it, ended up sideways, and went back and fixed the problem.

Most of us spend a lot more time figuring out engine upgrades, heck, even transmission upgrades, than brakes. But your brakes are the most important part on the car.

If your car doesn't stop right during an emergency stop- FIX IT.

Smaller rear wheel cylinders will help. Making sure you're running the same material on all your brakes (semi-metallic is best) is important too. And if that still doesn't fix it, then add that proportioning valve. Assuming, of course, that all the brake hardware is in good working order.

As I mentioned already, I run the later Mopar 11.75" front disks and 11x2.5" rear drums on my Challenger, and will on all of my A-bodies as soon as they're on the street (already have all the parts!). The braking force seems to be much better balanced "out of the box" than the 10.98" rotors and 10x2.5" drums. I still run the larger 15/16ths rear wheel cylinders, but with a 15/16th's master cylinder, full stainless lines, and all new hardware, pads and shoes. I also run drilled and slotted front disks. Tires make a big difference too, if you're running a staggered set up that can make a difference, as well as tire choice. The 275/40/17's I run all the way around make my brakes work for their money.

Regardless of your set up, spend the time to get it dialed in right. Brakes are one of those things that should only be stock on a museum piece. If you drive it, you should be able to stop it.
Click to expand...

Thanks for the lecture but it depends on your use of the term -

I was using the term "problem" to describe a "breakage" or "mal-function"....He doesn't have one.

What you describe as a "problem" is what is typical of the factory system.

I would call that an "issue" that is not specific to his car.

FWIW - The real gist of my post was simply trying to prevent the OP from going on a wild goose chase looking for breakages or faulty components when we all know its just a crap system until you mod it.


Anyhow - its just semantics...I'm sure we'll both get over it ......
 
check to see that the brake shoes are installed properly, there are primary and secondary brake shoes the primarys have more lining on them and always go to the rear of the car, if installed impropperly ie: two primaries on one side two secondaries on the other, primaries installed in the front will cause this, if you added the disk brakes to this car without the kelsey hayes proportioning valve will also cause this, when converting to big bolt disk brakes you need the proportioning valve that goes with that setup and may also need the adjustable proportioning valve as the a bodies are light in the rear early a bodies are definately prone to this problem with shorter wheelbase and lighter overall weight......just sayin from experience, when dealing with brake systems it's better to test them to the fullest in controlled manner than to find out later that there is a problem, I raced stock cars so we had to know what the car would do to gain that advantage...actually a larger wheel cylinder on the rear will have less braking affect as more fluid required to move the pistons inside the wheel cylinder.....Just Sayin
 
waggin said:
actually a larger wheel cylinder on the rear will have less braking affect as more fluid required to move the pistons inside the wheel cylinder.....Just Sayin
Click to expand...

It will take more pedal travel, you are correct in that respect, but larger wheel cylinders on the rear will apply greater force to the shoes which will make the problem worse. Its well known that smaller wheel cylinders apply less force on the shoes.

A smaller bore master cylinder however applies greater pressure.

And these brakes are crap as they come from the factory. You have to start with smaller 7/8" rear wheel cylinders, 15/16" master cylinder, and proportioning valve. Then make sure everything is in tip top shape. Then you might be able to slow the car down without the rear wheel skid.

I've been messing around with mine since '79. Back then the car was all original in good shape and it was an absolute death trap.
 
You must log in or register to reply here.
-

Back
Top