Rear tires locking up early

-
Map63Vette

Map63Vette

Well-Known Member
Joined
Apr 19, 2010
Messages
1,850
Reaction score
212
Location
Lawrence, KS
I noticed the other day when I had to get on the brakes a little bit in the wet that my rear end was locking up before my front. I had this happen one other time during an emergency braking moment on the highway that made me review my proportioning valve setting, but even after changing it I seem to have the same issue. Car is a 67 Dart with E body front brakes (~73 Challenger by castings, don't know rotor size off the top of my head) and something like 10" rear drums. I have a Dr. Diff proportioning valve in it and a master cylinder out of a gen 1 Viper (1 1/6" bore). After the first issue on the highway I looked up the valve and thought I adjusted it to the "softest" setting (knob unscrewed), but I'm wondering if that was correct or if something else might be going on. I might be able to unscrew it a little more, but it was getting so loose I was almost worried I was going to screw the whole thing out of the block (if you can even do that). There is very little resistance when turning the knob at this point, so I can't imagine there is much if any preload on the spring. Pads are reasonably fresh all the way around (changed in probably the last year or two at least) and I'm pretty sure they are all of a similar "blend", ie. the same brand and application (daily driver).

So with that said, did I just adjust my valve the wrong way or what should I be looking at next? I might try doing some testing on dry pavement with the current valve setting, but I'm wondering if I should be looking at rebuilding/replacing some stuff as well.
 
Big.J.007 said:
What size rear wheel cylinders?
Click to expand...
Hmm, that's a good question. I want to say 15/16", but that's just a gut feel I have and what I think I remember seeing on the package of the last one I bought. Don't actually know for sure.
 
The 15/16 typically provide too much rear brake bias. Even with a prop valve, its best to use a 7/8" rear wheel cylinder


Map63Vette said:
Hmm, that's a good question. I want to say 15/16", but that's just a gut feel I have and what I think I remember seeing on the package of the last one I bought. Don't actually know for sure.
Click to expand...
 
You may have adjusted the PV the wrong way. It would make more sense that you would adjust it closed to reduce braking to the rear, not adjust it open.
 
I'll have to look at the valve again. I just looked it up online and found the explanation I was looking for I think. There is text on it that says increase and decrease, but I didn't know if that meant increase and decrease pressure, or increase and decrease proportioning. Sounds like it refers to the actual pressure control, so I'll have to see which way I have it turned.
 
You might have to find a secluded area to test the adjustments on that proportioning valve (panic stops to see which end locks up first). When doing those tests also make sure the rear drums are adjusted first by hitting the brakes a few times while driving in reverse.
 
[1] An out of round drum will also cause premature lock up. If one side locks up, swap the drums to see if the problem moves.
[2] Anything that causes the brake shoes to rub on the drums changes the friction value of the lining & can cause lock up. Check that if you have automatic adjusters....that they are working, not binding. Also check the park brake cable/linkage for binding.
 
Are you sure the fronts are working?

I run the KH 4-piston front brakes, and 10x 2 drums ( which IIRC use 1.75 shoes.) on the rear with 15/16 wc's, and No proportioning whatsoever. and with a 15/16ths M/C, with Power assist.
BFG 235/60-14s on the front.
BFG 295/50-15s on the back.

The rear brakes really hard, but with 295s, not too hard. Just right, IMO. It worked with 275s as well, no drama.
 
All the way "loose" on the adjustable prop valve typically means its wide open- no proportioning.

If you adjust it in a few turns you should reduce the pressure to the rears, which should cure the lock up.
 
AJ/FormS said:
Are you sure the fronts are working?

I run the KH 4-piston front brakes, and 10x 2 drums ( which IIRC use 1.75 shoes.) on the rear with 15/16 wc's, and No proportioning whatsoever. and with a 15/16ths M/C, with Power assist.
BFG 235/60-14s on the front.
BFG 295/50-15s on the back.

The rear brakes really hard, but with 295s, not too hard. Just right, IMO. It worked with 275s as well, no drama.
Click to expand...
I'm not actually sure if the fronts are working at 100% efficiency. The pads do wear and I have brake dust up there, but it's possible the calipers are really sticky and just not putting the pressure into the pads. I thought I rebuilt them years ago, but honestly can't say for sure (it's basically one square ring to rebuild them, so easy enough to do again).

I've got the same size tires all around (235s or something like that), so there shouldn't be any bias caused by tires either to help eliminate that. I did double check my valve and had quite a bit more rotation left in the "decrease" direction than I thought, so adjusted it full "decreased" for my commute home yesterday. Gave the pedal a pretty weird two stage feel that got a little better by the time I got home, but it does still act a bit funny. Pedal was pretty stiff right off the top of the travel, would go down a little ways with moderate braking but nothing special, then hit a second stiff point where it felt like I threw an anchor out the window. Screwed it in a turn or so when I got to work this morning, so will see what it feels like going home today. Maybe if I get the chance I'll try to really hammer them on a clear stretch to see what happens.
 
Was your 67 Dart born with all drum brakes?
Does your dash have a working "brake warning light", and
if yes, is it on all the time?
How is the fluid split for the front brakes? I'm assuming there is some kind of Splitter or a Y-block. But there should be a pressure differential valve somewhere with an electrical switch and wire connector on it, to ground your dash-light. If you have this switch, you will have to prove that it is open-circuit to ground, which is the normal position, cuz if it's not, then it has shut off one of the dual-brake systems. When it does this, it is supposed to turn on the dash-light. This valve, if a MOPAR valve is supposed to be self-centering, but with age, they get gummed up.

Is the fluid level in the reservoirs topped up?
Did you bench bleed it? IE is the hydraulic chamber between the two power-pistons full of fluid?
Are the lines properly routed? IE, the chamber closest to the rad, drives the rear brake system.
Is your pedal properly parked at the top of it's stroke?
Does it have about 1 inch of travel before it gets hard?
Is your aftermarket P-valve the ONLY valve in the System?
Where exactly did you plumb the P-valve in?



IMO
I would get the front wheels off the floor, spin them up, and see if the brake pedal will stop them, and how much pedal pressure it takes.

Images;
This is the PROPORTIONING valve that would have been on a disc-drum car.
1738953526111.png
this is a drum-drum valve, without proportioning.
1738953346049.png

1738953859213.png
in the last image, you can see how the little shuttle-valve can work. In this valve, it appears that the shuttle does not have enough travel to completely shut the failed system off. This device only has one job, which is to turn the dashlite on, in the event of a pressure loss on one side of the system.
 
Last edited:
Pretty much the only stock parts of my brake system left are the lines. The front and rear brakes were already swapped when I bought the car, but I would guess the car was originally drum all around. That being said, I'm running a modern master cylinder with fully isolated front and rear circuits (at least the plumbing external to the master). I come out of the rear port of the master to a tee that splits to the front brakes, and out of the front port on the master into my aftermarket proportioning valve which then goes back to the axle and splits at the tee on the housing, so no distribution valve or warning switch left in the system. I also have a line lock solenoid plumbed into the front brakes, but it has no power at the moment as I don't have the switch installed yet.

Good idea on getting the wheels off the ground and spinning them though. That should be easy enough to test to see if/when the fronts are engaging. I bled the whole system a couple of times since the master was replaced, so pretty sure it's all good. Generally speaking, the car stops fairly well. It's not like it doesn't have brakes, more just that they are locking in a funny order, so I'm thinking I'm leaving some stopping performance on the table and would like to make sure I'm getting the most I can out of them just in case I need it all. Having locked them up twice now it's just at the top of my mind.
 
Read post #8 again.
Before you can cure lock up, you need to find the cause? If the drums are old, they are more likely to be out of round. No PV is going to fix that. If the drums have been machined, it makes them thinner....& more likely to go out of round. The drums are easily removed &............taken to a brake shop for checking. The drums could be machined if still in spec to clean up the surface....& you have eliminated one possible cause of the lock up. Not expensive. I had a pair of drums machined recently to clean them up, cost $30 ea.
 
This is a little known part for some reason, but it helps on both drum and disc rear brakes by cushioning/dampening the initial pressure spike of a quick brake application.

As mentioned above, using the correct sized rear wheel cylinders is the first step. Along with following the other great advice listed above.

Here's a picture and link to the anti-lock device I've been using with great results on my rear disc set-ups.

www.larsenracingproducts.com

Lock Resistant Brake System (LBS)

Ensure maximum safety and control with the Lock Resistant Brake System (LBS). Engineered to prevent wheel lockup, this advanced braking system provides superior stopping power and stability, ideal for high-performance and everyday driving conditions.
www.larsenracingproducts.com www.larsenracingproducts.com

Rear suspension.jpg


Brake anti-lock 1.jpg


Brake anti-lock 2.jpg
 
Bewy said:
Read post #8 again.
Before you can cure lock up, you need to find the cause? If the drums are old, they are more likely to be out of round. No PV is going to fix that. If the drums have been machined, it makes them thinner....& more likely to go out of round. The drums are easily removed &............taken to a brake shop for checking. The drums could be machined if still in spec to clean up the surface....& you have eliminated one possible cause of the lock up. Not expensive. I had a pair of drums machined recently to clean them up, cost $30 ea.
Click to expand...
The drums are fairly new. I know I have replaced them since I've owned the car, so they aren't entirely unknown at least. I think I need to just go do some testing on an empty stretch of road first. Easy enough to set the valve to a couple different positions and see if it makes any difference. That would help determine where to focus on next and wouldn't really take any work to do.

The pedal does have a weird feel after my adjustments though. It has a strange stiff initial hit, but then seems to fall under my foot under a constant pressure before getting stiff again. I'm wondering if I need a residual pressure valve or something as I'm pretty sure my modern master cylinder isn't going to have one built in if that's a drum brake thing.
 
This is not a residual line pressure problem. Many cars did not use a RLPV & many cars that did had them omitted in later years. New anything means nothing these days. I recently had to get a new trunk lock for my GTO. I got the genuine AC Delco, which was about double the price of others, believing I would get a made in USA piece. Nope, made in China....

Your new drums were likely made in China, still worth getting them checked.
 
Lots of great info on this thread. Drum brakes are referred to as self-energizing, as the shoes wedge into the drum when applied. Dr Diff said adjusting the front to rear brake bias is best done by altering the size and thus clamping force, typically of the rear wheel cylinders.

Jack the car up, put it in neutral and spin the rear tires. Do the pads contact the brake pads at all while you turn them? Do they only contact at a certain point? If they do, this can indicate warped drums, as the pads generally remain fixed when not applied.

Have you adjusted the brake pads properly? Being sure to hit the brakes between adjustments to center the pads and recheck the adjustment. Do you ever back up to 10 or so, and hit the brakes with medium or more pressure to allow the self-adjustment feature to work.

How is your parking brake working?
 
check for groves in your backing plates too the shoes can hang-up on them then jump under heavy braking
front pads that are too hard need heat to grab so if you quick hit the brakes rears might grab faster
do a couple of stops get them hot and see what happens
 
I'm beginning to wonder if the "spike damper" valve might be the answer after all. When doing some quick brake application tests, I noticed that the rears seem to lock up easy if you really whack the pedal quickly but not even necessarily that hard compared to a smooth but firm application. I'll have to check my shoe adjustment too as I was talking with someone at work and he brought up a good point that my funny "two stage" pedal feel might just be the rears taking some time to spread, so the proportioning valve isn't doing its job right off the hit. I know when I put them on I tried to adjust them to give some slight drag when turning things by hand and I do the reverse and brake thing a couple of times whenever I finish working on drum brakes.
 
What a lot of people do not realise:
- car has brakes on 4 wheels
- until the last of the brakes of the 4th wheel has applied, there is no real pressure build up. Not enough to lock wheels.
- shoe adjustment is not going to affect the fluid pressure that is being applied to the shoes via the wheel cyl
- shoes that have a lot of clearance cause the brake pedal to be lower. No braking action will take place until the shoe contacts the drum
- on the other hand, tight brake shoes that are rubbing on the drum heat up the lining & can make the shoes 'grabby'.
 
Bewy said:
Read post #8 again.
Before you can cure lock up, you need to find the cause? If the drums are old, they are more likely to be out of round. No PV is going to fix that. If the drums have been machined, it makes them thinner....& more likely to go out of round. The drums are easily removed &............taken to a brake shop for checking. The drums could be machined if still in spec to clean up the surface....& you have eliminated one possible cause of the lock up. Not expensive. I had a pair of drums machined recently to clean them up, cost $30 ea.
Click to expand...
Well, you don't live in the US... Here you'll have a hard time finding a shop that still has a brake lathe... To many lawyers.... I tried to get a pair of rotors turned fifteen years ago & all the shops that use to do that sort of work had no interest in getting involved...

These days I own a lathe, not a brake lathe so it takes a little ingenuity but it works...
 
I haven't had more then 2 drums or rotor in 20 years
The only one I know that has a lathe is napa
As long as its still in spec they can be cut
Problem is new cars have high metallic compounds that ware out the rotors as fast as the pads ..or around here they rust
So cutting rotors have gone the way of the carb
 
Bewy said:
What a lot of people do not realise:
- car has brakes on 4 wheels
- until the last of the brakes of the 4th wheel has applied, there is no real pressure build up. Not enough to lock wheels.
- shoe adjustment is not going to affect the fluid pressure that is being applied to the shoes via the wheel cyl
- shoes that have a lot of clearance cause the brake pedal to be lower. No braking action will take place until the shoe contacts the drum
- on the other hand, tight brake shoes that are rubbing on the drum heat up the lining & can make the shoes 'grabby'.
Click to expand...

I think that's only partially true. Most cars have independent front and rear circuits for just that reason. You don't want to lose all of your braking if you have one bad corner. I'm not sure how exactly that plays with the pedal though as the master cylinder is still one linked piston as far as I'm aware, so seems like there would have to be something in there to allow one circuit to still build pressure before the other.
 
You must log in or register to reply here.
-

Back
Top