The Stovebolt Site Forums The Shop Area Driveline Brake lock up and proportioning valves

My ’53 3600 has (by prior owner) a power brake conversion with dual master cylinder, part # KORN775 from Brothers. The past owner assured me that all of the wheel cylinders were either new or completely rebuilt. I have carefully tried to adjust the brakes per the shop manual but when I apply the brakes hard, the booster really works but usually locks up one rear wheel. I turn the adjusting wheels until the shoe drags slightly then back off 5 notches to let the wheel turn freely.
Is there a proportioning valve kit for drum/drum applications? Brothers has them for Disc/Drum or Disc/Disc applications but not for drum/drum. Any suggestions?
Thanks
John

Hy John Hill, rear wheel lockup on hard braking has been a problem for eons on pickup trucks, the weight shifts to the front, actually removing some of what little weight is over the rear wheels of an unladen pickup truck. Many cures have been tried, including an adjustable proportioning valve mounted between the frame and rear axle, the more weight in the box the more rear brakes you get. If the problem isn't causing you to lose control, and only occurs on hard braking, you could live with it, or see if you can mount an adjustable proportioning valve on your truck, like the one I described above, hope that helps P.S. the problem will be magnafied by slippery conditions.

....just a thought here now, just a thought.

Would it help to put a residual valve on the line going to the rear breaks so that the brakes would come on a little "quicker" before the front gets all loaded up and nose dives letting the rear wheel slide??
I know, one would think the residual valve would make it worst but think about it.... if the rear started coming on a little quicker before most of the weight is being stopped by the front wouldn't it help?????

...there I go, thinking again!!

My suggestion is to learn to apply as much brake as you need to in order to bring it to a stop. You need to remember, you don't have ABS. This is how I learned to drive with air brakes 50 years ago. Rear end lock up has always been a problem. This is why tire manufacturers recommend you put your best tires on the rear.

I know on my 1 Ton, the rear brakes are much larger and on a really slippery surface, they could be a problem. Brakes weren't as advanced in those days. Proportioning valves do help but I have no idea if they make them for a drum/drum setup. On some mini vans, they use a variable proportioning valve that is controlled by the ride height in the rear. They work great when new but as the springs settle, they provide more rear braking because of the change in the spring height, which of course is not what was intended.

I have done as "Crenwelge" suggested and have "mastered" the fine touch as best as I can. Moderate speeds on dry pavement is fine. If I had to do an emergency stop she will break loose. I guess that even though I am a "senior citizen" and have driven with all drum brakes for ever, I have become accustomed to the disc and ABS. Even one of my motorcycles (1800 Honda Goldwing) has ABS.
Thanks for the replies.
John

Have you checked the free play on the brake pedal? This can cause more than one brake to lock up.

NOTE: I mentioned above perhaps using a residual valve and a good friend and fellow bolter informed me that the old drum/drum master cylinders had it "built in". Soooo much for my "thinking/wondering"!!
I imagine the rssiduals would be nice if converting to a modern dual master cylinder. There I go "thinking again".

Monroe,
Are you talking about the "Toe board clearance adjustment"? My manual is only through '52 and my truck is a '53. It calls for 13/16" clearance under the floor toe pan and the top of the pedal arm.
John

This is MacX's dad Keith making this post... And I'm a novice to these old trucks, so please bear with me... So far I have less than 1 hour of cumulative drive time in the 53 that Mac and I have just got running this summer... And the brakes work great so far, but we put a lot of hours since October making the brakes work, keeping as many of the original parts in place as we could along the way.

I'm confused about a couple of things in this thread, though.

John, is the MC for the power brake conversion mounted on the firewall, or still in the original location under the cab? Isnt the height of the MC relative to the wheel cylinders going to have some bearing on the residual pressure (gravitational) in the brake lines? I guess I should look up the part number you listed and make sure, but I assume its a dual chamber MC so it has 2 outputs, and each one splits evenly to the 2 wheels it serves?

Here's the part that really is bugging me, though. You say you only have trouble with 1 wheel locking up in the rear, and not 2?

If it was both wheels, evenly, then I would be satified that the issue is a balance between the correct residual valve pressure and even distribution valve/blocks, and driver management to regulate just the right pressure...(50's trucks are new to me, but I cut my teeth on 60's muscle cars when I was in my teens, so not locking up was a skill, but getting even brake pressure on the left and the right, was a critical mechanical issue ...)

So, in my experience, 1 out of 2 wheels locking up in the back is a sign of trouble somewhere... brake shoes that have different surface conditions (greasy, or sticky, or thinner/thicker left vs right...), or leaking wheel cylinder, or obstruction in the brake line.... something is wrong if you're not locking up both rear wheel evenly, isnt it? Whatever this fault is, if it was occuring on the front, it would be dangerous, because the truck would pull sharply to the side with the harder braking, and then emergency stops are extremely dangerous to manage...

My driver side rear brakes were locking up on my 53/3600 I changed out the rear wheel cylinders and have not had a problem(knock on wood)Found the parts on the internet paid 70 bucks for them.
Good Luck

John, the problem could be that the mix of new vs rebuilt wheel cylinders that you said the PO put on, is a new one on one side, and a rebuilt one on the other side. A combination like that is very likely to create an imbalance in performance at equal brake line pressure.... You dont want to correct that problem with a proportioning valve between left and right, you want to correct that problem by putting in a matched pair of wheel cylinders ...

You will find that these trucks have more braking power in the rear than they need, when not loaded. As the rear wheels start to break traction, one will go before the other. If you look at some of the hotrod magazines or Classic Truck, you will find that they sell an adjustable proportioning valve. Since I have never used one, my advice ends there.

I had the same problem with the same system and it turned out to be incorrect bleeding of the system. i purchased a proportioning valve and when they went to install it no fluid came out of the right brake line. i still installed it. no problems in 4 years since. ron

Hello Gentlemen, let me add my $.02.

I remember reading a few years ago in one of the truck magazines, where a project truck had a similar problem. They put a dual master cylinder on a 1956 TF truck. Granted, is is not a 1952-53 AD truck, but the concept may be similar. Anyway, they installed the dual MC, and had problems with one of the rear wheels locking up all the time. Turns out, the way the factory metal brake lines were run, and they verified this in the applicable factory service manual, was that one of the rear wheels was on a line all its own.

Meaning, when they plumbed in the new MC, both front wheels and one rear wheel were run off one chamber off the master cylinder, and the other rear wheel was run entirely off the other chamber. I don’t remember the specifics of how it was plumbed or anything like that. However, I recommend you hand over hand check the connections of all the metal brake lines and see how it is plumbed. If you do have this problem of one of the rear brakes is plumbed all by itself, you can install a Tee in the line to isolate the front from the back. This is not specific, you will need to figure the correct plumbing out. However, I think you can get the gist of what I am saying.


If you have access to a brake pressure gauge, check the pressure at each wheel cylinder with the brakes applied, and see how they compare.

Good luck!

Seth K. Pyle

Good point Seth makes but he got it backward. One line feeds one front and the other feeds the other front and both rears on a '56. I don't think that applies on any ADs though.

Will this single-wheel-lockup be minimal, and alternate sides, if the brake system itself is flawless? Or is there something systemic in the design that will favor 1 wheel when excess braking is applied under a light load?

I'm just guessing, but it seems like it shouldnt take too much more braking to get the second rear wheel to lock up?

My truck does not do this, but I have the original single resovoir MC, and the 1 wheel/3 wheel brake line distribution that Cletis decscribes, so I dont think the same physics apply to my truck that John Hill is having.

John, your original post sounds to me like it is possible you havent given the brakes a close inspection yet, and are just going by what the PO told you about their repair/condition.

You may not be able to eliminate the rear-wheel lockup problem you are currently having, but my instincts tell me it's worth at least a little more diagnosis to make sure that your case is "normal", and not a sign of a more serious problem, like a line blockage to the wheel that is not locking up...

When my son and I were doing the brakes on our truck, we had the whole truck very securely sitting on jack stands and wooden blocks so that all 4 wheels were safely off the ground. When we were "bench" testing the brakes, Mac would gently apply the pedal while I turned 1 of the wheels. As soon as I felt some resistance, Mac would then hold the pedal steady where it was, and I would check the other 3 wheels to make sure they were getting equal resistance. We worked our way from low resistance, to lock-up, and I used a tire iron to try turning the wheels to make sure I could not turn them. The natural equalization of the line pressure from the single cylinder MC did most of the work, and we only did minimal adjustment of each wheel until I felt comfortable I had acheived my goals of an even braking system.

Now that the truck is on the ground and we ~can~ road test it,
under road conditions, most of the time I have very even braking,
but I have to admit they are not yet perfect... Occasionaly I get a sharp pull to the left from the front drivers wheel.... This is the 1 brand new wheel cylinder in the system, and it is the 1 wheel with it's own dedicated brake line coming from the distribution block from the MC. So, I am still "in the process" of diagnosing and tuning the braking on this old truck.

I had a thing with a car locking up the right front but not releasing turned out to be the rubber line went bad fron the inside. replaced the line and low and behold brakes? t mix

If only one rear wheel is locking up the problem most likely lies with that wheel. I had this problem with my truck, the right rear would lock up on a gravel road. Adjusted the brakes as closely as possible, it would still lock up. From side to side I swapped the drums, then the shoes and finally the return springs. When I swapped the return springs the left rear would lock up. Those springs were weak and when pressure was applied would allow the shoes to contact the drum before the other side.
John
Milford, MI