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Old 05-07-2007, 11:15 AM
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Rear wheel lockup

My 41 Pontiac is locking up the rear brakes, again. Had this trouble before and added an adjustable proportioning valve to the rear. Took 8 out of the possible 11 turns to balance the system. (original single stage master cylinder).
Just added a dual master cylinder and power booster, same 4 wheel drum brakes. New master has separate line for each front wheel so I expected to decrease the adjustable prop valve to the rear. Instead, the problem is worse. Back brakes lock up quick. I adjusted the valve 100%, still locking the backs first.
now what??

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Old 05-07-2007, 12:34 PM
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I have fitted a Chrysler 1981 New Yorker drive train and rear axle, complete with brakes, to my 1939 Hudson Coupe.

The Service Manual for the Chrysler brakes says that the Prop Valve is also a hold-off valve, which delays the operation of the front disc brakes, which are instantaneous, to allow the pressure to overcome the springs in the rear drums and apply the rear brakes first. This valve keeps the output pressure to the front brakes in the 3-30 psi. range until the hold-off pressure is reached (117psi).

So your rear brakes coming on first would appear to be correct.

The only thing I can suggest to check is the brake shoes themselves, you have a Leading shoe, and Trailing shoe on each wheel, (sometimes called Primary and Secondary Shoes) check that the shoes are fitted in the correct manner. If the Trailing shoe has been fitted the same way round as the Primary shoe, that could cause a lock-up going forward and a poor parking brake going backward.
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Old 05-08-2007, 09:41 AM
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re: Rear wheel lockup

Mosstrooper - Agree with your comments. Here's what I think I know....
4 wheel drum brakes. Original 41 pontiac front brakes, 74 Nova rear end/rear brakes. This was a single stage system, same size lines to all 4 wheels. 1 inch front cylinders, 7/8 rears. 11x 1 3/4 front shoes, 9x2 rears.
The smaller rear cylinders should react first (rear) but the larger (fronts) would be stronger and generate more stopping. I also have 225x15 tires in back and 215 x 15 in the front, making the backs a little harder to lock up.
Conversely, it is a coupe with more weight in front and a slight rake, add in the weight transfer, and the backs locked up first. A proportioning valve fixed that. So I was getting maximum stopping from the system, but it was still poor. I just added a 2 stage MC and power booster. Had to reduce the line sizes from 1/4 to 3/16 to match the MC, but it has 2 front lines (one to each wheel) and one rear line, to a split block. (replaced hardlines all the way to the new flex lines. ) I expected the rears to still engage first, but the front to have increased stopping power since I doubled the amount of fluid transferring. The brakes are much stronger, with an easier pedal, so that part is fine - but the backs lock up fast, which is the opposite of my expectations. I'm awaiting parts to complete the job, which is 52 Pontiac front drums, shoes and wheel cylinders. They are 2 1/4 wide drums with 11x2 1/4 shoes and 1 1/16 wheel cylinders. All of which should add up to even better front stopping power. Logically this means I should be able to lock up the fronts much easier than the rears - so I can remove the reduction to the rear wheels and have better brakes everywhere. I am stumped by the results I got, and wonder if I am missing something. I don't want to get to the point where I have to virtually block off the rears to balance the system.
Is there something wrong with my analysis/engineering, or should I be looking for something in the rear system that is causing premature lockup, not the design itself? - no, the shoes are not in backwards.
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Old 05-08-2007, 11:08 AM
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A few questions as the answers might shed some light on the problem. What was the original master cylinder from? What is the new master cylinder from? What are the bore sizes of both of the master cylinders?
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Old 05-08-2007, 12:00 PM
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Having front drums makes a difference to what I said before, the last car I had with front drums (57 Cadillac) was a pain to set up properly. The front drums were adjusted using a set of feeler guages, slipped into the drum through a slot in the drum itself.

I still have the Caddy manual and I can look it out if it would help you, If I remember correctly, the gap between the Shoe/Drum at the top had to be 4 thou. and at the bottom had to be 2 thou. anywhere outside these measurements slashed the brake effect by more than half. And since we have both shoes leading on the front, the measurement had to be reversed for the other shoe, if you can follow what I'm saying.

I'm tending to think along the lines of your rear brakes being good, and the fronts not good enough.
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Old 05-08-2007, 12:38 PM
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Re: rear wheel lock up

Homebrew. The original master cylinder was the orig., 41 Pontiac single jar. The new one is from a Chevy/Geo Metro Ė picked it for the small 7 in. booster size, and it powered a 4 wheel disc system. My cousin (the junk yard guru) said they worked great on Mustang, Fairlane, streetrod power brake conversions. Donít know the actual bore size on either. I didnít grab the original Proportioning/Metering valve cause it was designed for discs, and a completely different car.
Mosstrooper. Right again. Took me a while to figure out how to do the feeler gauge through the slot adjustment, but it works very well.
Hope you are right about the rears just being better. The next step would fix that.

You guys see anything wrong about my understanding of the things that affect braking and balancing the system?
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Old 05-08-2007, 02:55 PM
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I don't know if you've checked this, but are your rear shoes clean? If there is any oil or other contaminants on them, they will grab and lock up suddenly. Even with a new set of shoes, if you got them dirty while installing them, you could have this problem just the same. And dirty shoes can not be cleaned once the dirt or oil is ground in. I had this problem with my Ranger pick up. I would clean the shoes, and all would be fine until the shoes warmed up and the contaminants bled out again, making the brakes grab.
Maybe you've checked this already, but it may be worth a try.
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Old 05-08-2007, 05:10 PM
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Everything you have talked about seems OK. The only difference after you finally got the rear brake problem solved with the proportioning valve is the master cylinder and brake line size. I don't think the line size should make any difference but I could be corrected on that. What is left is the power master cylinder which is originally made for four wheel disc brakes and which should easily have enough pressure to operate your drums - maybe too much???? Some disc brake systems require as much as 14,000psi for proper operation while a drum system would very seldom require 1,000 psi.

When you said two stage I assume you mean dual master cylinder, is that correct? I know that there are two stage cylinders for clutches but I have not heard of them for brakes.
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Old 05-08-2007, 06:39 PM
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re: rear brake lockup.

I just found out the metro had drums in the rear. Got bad info somewhere.
Could be I just have too much pressure in the lines, and the back brakes are definetly better than the fronts, so it may be OK when all is done. It's just that I had it balanced once and expected it to go the other way.
I'll look for contaminates. Not that big a deal to turn the rears again and slap on new shoes, or can I just replace the shoes? With brakes, I prefer to do a total job, not peice meal, but they were turned not too long ago.

Line size is smaller, and the master looks small. Wondering if the 1 in. cylinders in the front, and the 1 1/16 I have planned, are too big for the master. How would I find the specs, bore and stroke, for the MC and wheel cylinders to calculate sufficient fluid volume? I just assumed disc brakes, even small ones, would take more fluid than cylinders.
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Old 05-08-2007, 07:18 PM
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no prop valve needed for 4whl drums, remove it and replace with a short line, and see what happens. also, you need to put a 10 # Residual preassure valve in the front brake circut as, the springs on the shoes can actully push the wheel cylinders in too far, and make your front brakes sluggish because they have to travle, the 10 # RPV ( which is built into drum/ drum M/C's ) takes care of this. and it seems, this may be your solution. Willwood, and Speedway motors, also Summit, sell 10 # RPV's for about the same cost, Willwood is the best.
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Old 05-08-2007, 10:02 PM
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re: rear brake lockup.

Sorry Matt, but that doesn't make sense to me. With 4 wheel drum brakes, Why would the return springs on the front create a different condition than the return springs on the rear? Also, a residual valve is mainly to hold fluid in the lines when the mc is below the cylinders, don't know of them being used when the MC is up on the firewall. Lastly, proportioning valves are needed when you have different, unmatched components in street rods such as different tire/wheel sizes front to rear, mismatched shoes/drums/wheel cylinders front to rear, and raked chassis creating unbalanced front to rear weight bias. Man, If I am wrong about all of the above, I am in deep doodoo.
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Old 05-08-2007, 10:56 PM
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Now that we know that the master cylinder was for a front disc and rear drum setup we have another possiblity. If the master has a built in metering valve to the front disc brakes (most disc/drum setups use a metering valve to hold off the front discs until the rear begin to apply) then the rears would grab much sooner thanthe front brakes. In fact it may take a long time before the front brakes actually energize if they ever do. I think you really need a master cylinder that is meant for four wheel drum or four wheel disc brakes.
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Old 05-09-2007, 08:05 AM
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re: rear brakes lock up

AAAH, that makes sense. I though I was getting an all disc setup, but the disc/drum could be an issue. I'm doing research on this MC and the proportioning/metering valve that was in the original system. Now that I know what to look for i'm ready to move on. I'll get back to you in a couple of weeks with the results. Thanks for your help.
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Old 05-09-2007, 10:21 AM
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It may be worth trying to swap the lines at the master and than readjust your proportioning valve to reduce the front brake pressure with it.
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Old 05-09-2007, 11:56 AM
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Quote:
Originally Posted by oldguy829
Sorry Matt, but that doesn't make sense to me. With 4 wheel drum brakes, Why would the return springs on the front create a different condition than the return springs on the rear? Also, a residual valve is mainly to hold fluid in the lines when the mc is below the cylinders, don't know of them being used when the MC is up on the firewall. Lastly, proportioning valves are needed when you have different, unmatched components in street rods such as different tire/wheel sizes front to rear, mismatched shoes/drums/wheel cylinders front to rear, and raked chassis creating unbalanced front to rear weight bias. Man, If I am wrong about all of the above, I am in deep doodoo.
2# residual valves are used to keep fluid from draining back, but 10# valves are for as I said, and are buit into drum brake circuts, your disc drum m/c has 1 for the rear but not the front ( rear works properly, locks up, and front eventully catches up ).
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