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#1 |
Registered User
Join Date: Feb 2009
Location: Choctaw, Ok
Posts: 184
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1968 C10 CST DRUM/DRUM Proportioning Valve Problem
Just purchased a 68 C10. Driving home from work today the rear brakes started locking up and not disengaging. It has DRUM/DRUM set up with power brakes and a proportioning valve. It sounds like the proportioning valve is bad to me. I can't find one online for DRUM/DRUM. Can anybody point me in the right direction?
The rear brakes disintegrated and were locked in the braking position until I released the bleeder screw on the wheel cylinders. I checked the wheel cylinders and they both move in and out freely, making me think proportioning valve. Any words of wisdom? I am also going back together with all new brake parts, but can't find the valve anywhere for DRUM/DRUM. Attached picture shows the assembly in question.
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choctawchevyclassics ![]() Last edited by choctawchevyclassics; 07-24-2019 at 10:06 PM. Reason: picture |
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#2 |
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Location: Texan in Iowa
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Re: 1968 C10 CST DRUM/DRUM Proportioning Valve Problem
A 1968 drum/drum system would not have an actual prop valve, but instead has an electrical switch in that spot that lets you know if one of the systems (frt/rr) loses pressure. Sounds like your rear cylinders are getting pressure. Are you sure the shoes and other mechanical parts are installed correctly on the rear brakes?
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Ben '68 Chevy C10 Custom LWB 327/TH400 2nd owner '16 GMC Sierra 1500 SLE Dblcab 4wd 5.3 |
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#3 | |
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Re: 1968 C10 CST DRUM/DRUM Proportioning Valve Problem
Quote:
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choctawchevyclassics ![]() |
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#4 | |
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Re: 1968 C10 CST DRUM/DRUM Proportioning Valve Problem
Quote:
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choctawchevyclassics ![]() |
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#5 |
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Re: 1968 C10 CST DRUM/DRUM Proportioning Valve Problem
Yes. Cast iron, about the same size as a prop valve, and with a single wire connected to the bottom.
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Ben '68 Chevy C10 Custom LWB 327/TH400 2nd owner '16 GMC Sierra 1500 SLE Dblcab 4wd 5.3 |
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#6 |
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Location: Choctaw, Ok
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Re: 1968 C10 CST DRUM/DRUM Proportioning Valve Problem
So what I have pictured is just a switch? What would cause both rear brakes to start locking up at the same time all at once? It has been driving and braking fine until this afternoon. And the brakes are all installed correctly. Could the switch itself not allow a flow back after release of the brakes and keep the rear brakes engaged?
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choctawchevyclassics ![]() |
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#7 |
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Re: 1968 C10 CST DRUM/DRUM Proportioning Valve Problem
Below is what you should have. It is usually referred to as a distribution block. However, I doubt that would be your problem.
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#8 |
Who Changed This?
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Re: 1968 C10 CST DRUM/DRUM Proportioning Valve Problem
Most likely the flex hose at the rear axle has failed and isn't letting fluid return to the master cylinder when you let off the brake. Is it the original hose?
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~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA |
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#9 |
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Re: 1968 C10 CST DRUM/DRUM Proportioning Valve Problem
I have not checked that, yet. first thing in the morning.
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choctawchevyclassics ![]() |
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#10 | |
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Re: 1968 C10 CST DRUM/DRUM Proportioning Valve Problem
Quote:
I have had to replace several of the rubber brake hoses on my trucks
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Mark 72 c20 custom camper Husky edition, 66 SS396 Chevelle 1964 Hawk, 63 Avanti,62 lark 1969 AMX , 1968 c20 stepside ,85 K20 1977 Suburban sold 68 anniversary. |
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#11 |
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Re: 1968 C10 CST DRUM/DRUM Proportioning Valve Problem
I had a problem sounding like this when one of the pins that hold the shoes on the backing plate broke. The rear brakes would engage on and off on their own.
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Tony Nicholas @ Midwest Metal Masters 1953 Chevy 3100 392 Hemi w/727 & 9” Ford 1985 Chevy C/10 6.0L 4L80 373 locker |
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#12 |
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Re: 1968 C10 CST DRUM/DRUM Proportioning Valve Problem
wish it were that easy. All of the hardware is new and good. nothing broken or out of place.
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#13 |
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Re: 1968 C10 CST DRUM/DRUM Proportioning Valve Problem
i agree on the rubber line leading to the distribution block on the rear end. once its soft itll collapse on you also make sure theres no debris in the block at the rear end while you go tthe hose off
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#14 |
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Re: 1968 C10 CST DRUM/DRUM Proportioning Valve Problem
In the photo, one of the brake lines looks to me unusually close to the exhaust manifold - I think brake fluid boils at under 300F? Just an observation.
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#15 |
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Re: 1968 C10 CST DRUM/DRUM Proportioning Valve Problem
Good point! But then maybe both would do it, from the looks of the picture.
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~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA |
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#16 |
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Re: 1968 C10 CST DRUM/DRUM Proportioning Valve Problem
Are you looking at my picture in post 6 or the one the OP posted as an edit to his original post at the same time I posted? If mine, they are not close to the manifold, but they do look close in the picture. They are run down to, and attached to the frame just as the factory did it.
Last edited by FirstOwner69; 07-26-2019 at 10:56 AM. |
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#17 |
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Re: 1968 C10 CST DRUM/DRUM Proportioning Valve Problem
I was looking at the photo in post 6; and the OP's photo doesn't look as close to the exhaust manifold.
So it doesn't look good for my theory then. But maybe still worth considering thermal expansion of the brake fluid as a potential cause to be ruled out if somewhere along the chassis or crossover the rear brake line or lines run too close to the exhaust? |
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#18 |
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Re: 1968 C10 CST DRUM/DRUM Proportioning Valve Problem
>> thermal expansion of the brake fluid as a potential cause to be ruled out <<
If the fluid is overheated to the point of boiling, the opposite will happen. You would simply loose brake pressure. |
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#19 | |
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Re: 1968 C10 CST DRUM/DRUM Proportioning Valve Problem
Quote:
I agree. We had a bad experience with our 1986 dually crew cab with a 454. We took it to the Rocky Mountain states with the camper on board. Every time we'd climb a steep grade to a mountain pass (and there were many), we'd have to stop at the summit to let the fluid cool because we had no brakes. When we got home, I had the fluid flushed and a shield put on the exhaust down pipe which was close to the brake lines. It seemed better after that. That truck got extremely hot under the hood. |
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#20 |
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Re: 1968 C10 CST DRUM/DRUM Proportioning Valve Problem
If the rear brakes release when you open the rear cylinder bleed nipple the are only two possible causes:
The rear flex line between the chassis and rear end has failed and is not permitting fluid to return back to the master cylinder....replace the hose.... or The master cylinder has insufficient clearance on the booster rod that sits in the end of the master.....to check, loosen the master cylinder off the booster by about an 1/8" when the brakes are locked on....if they release, the booster rod needs to be backed down by a small amount.... If the rear port on the piston inside the body of the master cylinder does not come back far enough to exhaust, the brakes will lock on as you describe. ![]() ![]() ![]()
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#21 |
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Re: 1968 C10 CST DRUM/DRUM Proportioning Valve Problem
Just a thought also on a compound problem, are you sure the fronts are fully working and the rears are not locking up due to trying to stop the entire truck? I’m with the swollen rear hose, but never hurts to make sure you don’t have another issue making the other worse.
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#22 |
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Location: Ark City, Kansas
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Re: 1968 C10 CST DRUM/DRUM Proportioning Valve Problem
I've never seen a probpem with the distribution block on any all drum vehicle. Seen hundreds of brake hoses cause your problem.
I'd replace the rear hose. If the front 2 hoses have not been replaced in at least 10 years, I'd replace them some time soon, also!!! |
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