1968 C10 CST DRUM/DRUM Proportioning Valve Problem

[choctawchevyclassics]

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.

[ThreeQuarter]

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?

[choctawchevyclassics]

Originally Posted by ThreeQuarter

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?

All of the brakes are correctly installed and this does have a prop valve on it.

[choctawchevyclassics]

Originally Posted by ThreeQuarter

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?

Would this switch you are talking about look like a prop valve with 2 lines in and 2 lines out and an electrical connector?

[ThreeQuarter]

Yes. Cast iron, about the same size as a prop valve, and with a single wire connected to the bottom.

[FirstOwner69]

Below is what you should have. It is usually referred to as a distribution block. However, I doubt that would be your problem.

[choctawchevyclassics]

Originally Posted by ThreeQuarter

Yes. Cast iron, about the same size as a prop valve, and with a single wire connected to the bottom.

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?

[Steeveedee]

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?

[choctawchevyclassics]

Originally Posted by Steeveedee

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?

I have not checked that, yet. first thing in the morning.

[65standard]

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.

[choctawchevyclassics]

Originally Posted by 65standard

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.

wish it were that easy. All of the hardware is new and good. nothing broken or out of place.

[72c20customcamper]

Originally Posted by Steeveedee

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?

What he said^^^

I have had to replace several of the rubber brake hoses on my trucks

[michael bustamante]

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

[Gromit]

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.

[Steeveedee]

Originally Posted by Gromit

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.

Good point! But then maybe both would do it, from the looks of the picture.

[FirstOwner69]

Originally Posted by Gromit

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.

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.

[Gromit]

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?

[RichardJ]

>> 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.

[FirstOwner69]

Originally Posted by RichardJ

>> 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.

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.

[68Gold/white]

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!!!

[AussieinNC]

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.

[Madkidd007]

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.