The GMC CCKW

I am trying to repair the brakes on my 44 CCKW.I restored the truck years ago and I noticed I had a few leaks in the brake system this winter.The master cylinder was leaking as well as the hydro vac.I replaced the master cylinder with a new one and had the hydro vac (series two) rebuilt.Now when I pump the pedal to bleed the system all of the wheels lock up tight.The wheel cylinders are all good and the return springs are all in serviceable condition.I have all new steel lines and rubber hoses.The brakes worked fine before I replaced and rebuilt these parts.I suspect it may be a bad rebuild on the hydro vac,Because the wheels stay lock when I disconnect the master cylinder.They lock up with the truck not running and when I pump the pedal until I get pressure.The only way to unlock the brakes is to open a bleeder screw.I have not yet bleed the brakes as they lock up overtime I try.Does this sound like a hydro vac issue? I want to be sure before I send it back to the shop.

If all rods are adjusted properly and the hydro-vac is connected correctly....I would buy/build a simple line to bypass the Hydrovac and see if the lockup continues. This is the simplest and cheapest diagnostic and will unquestionably tell you whether the hydrovac is the cause or not.

It will require a double fitting line of proper length and a couple of double female couplers.

Let us know.

Bill,I did as you said,Now the brakes work fine.So,This must be a Hydrovac issue.

You my have the wrong master cylinder.

The M35 and CCKW have no check valve f I remember right

The M211 has the check valve

The booster has a check valve if I remember right.

You cannot have2 check valves....

Harold,

I agree with Dr. Deuce. If it was sold as an M-35 Master that fits the CCKW the addtional check valve is probably what it is.

The good Doctor had me remove mine prior to installation.

Yes I agree about the check valve.But,This is only a issue if you have a newer series 3 Hydrovac.I still have the original series 2.Also,If I disconnect the master cylinder the brakes are still locked.There is no pressure kept between the master cylinder and the Hydrovac,Only from the Hydrovac to the wheels.

I agree with you on the check valve,But this is only a issue if you have a series 3 type hydrovac.I am still using a series 2 type.This type needs the check valve in the master cylinder.

OK, if you shut the engine off and pump the brakes till there is no more boost, do they release then?

http://imageshack.us/photo/my-images/59/y9kp.jpg/
letter "J" -Check Valve.

Thanks for the help.The brakes lock up with the truck not running.So I do not think it is a vacumn issue.I have removed the Hydrovac and returned it to the rebuilders.I hope they can get it right.

http://imageshack.us/photo/my-images/580/kxqu.jpg/
I think you should check the STOP SCREW(0.020" clearance) under the master cylinder.

I had the exact same problem until I discovered that the fluid escape hole on the bottom of the brake fluid reservoir was rusted clogged! I burned a couple sets of brake shoes with this one! There is a smal 1/16" hole that needs to be cleaned out when restoring the M/C. If the hole is clogged there is no way for the fluid to return to the M/C and the shoes to retract to the resting position.

Knowledge is power!!

Another thread where the outcome is not reported and I would sure like to know what was found as the series 2 hydrovac on my buddies truck is doing exactly the same thing.
It is definitely the hydrovac that is holding pressure in the system downstream. The unit was professionally rebuilt prior to being installed and the series 2 doesn't have a check valve.

David