The GMC CCKW

Good evening from Belgium!

I have a problem with my 1943 CCKW. I recently bought it and after braking a couple of times (while driving), I notice that the rear (rear axle) right brake drum gets warm/hot. Also the brake pedal gets harder to press. When I bleed the rear right by opening the bleeder screw, the brake fluid shoots out, the brake releases and I can drive again.

So I took apart the brake cylinder, just to find out that this thing is completely new (the previous owner renewed it completely appearantly) and the thing moves just perfectly. Also the flexible brake line to the cylinder is new and free of any obstruction. I also checked the brake line from the T junction at the front rear wheel to the flex hose; this line is free from obstruction too.

So I assembled the whole thing again ... just to find out that during the testdrive, the brakes got stuck again (they had too since I didn't find any possible cause). I now however, also noticed the front drums getting warm! I guess that the rear right got warm first due to the fact that this one moves easiest and these linings will touch the drum first.

However, now I'm back to square one and the question remains: what's the cause of this? I've read that when ALL wheels get stuck, the problem will be an incorrectly adjusted brake linkage (between the brake pedal and master cylinder). But I've been thinking if there is a logical explanation for this to be the cause.

Lets say that the brake linkage indeed it is not correctly adjusted and that the opening (in the master cyl) to the brake fluid container in the master cyl is not open and the fluid can not flow back into the container. Now, if I brake, the pressure in the system will force the wheel cylinder to expand. If I release the brake, the cylinder will come back the same amount as I pressed it. This will only work if the truck is not driving, but only standing. Because if the truck is rolling, braking will create heat, warm up the brake fluid, make it expand and press even harder because the return opening to the container is not open ... ah, now as I'm typing and thinking, I'm beginning to understand

Ok, so I guess there e is one more check I should do (PLEASE correct me if I'm wrong!!!):
- Cold truck.
- Bleed the brake to make sure it's not under pressure.
- Idle the engine, don't drive!
- Press the brakes a couple of time.
- Lift wheel rear rear right.
- If it's NOT being braked now, the cause will be the master cylinder.
- If it IS being braked now, the cause is the hydrovac ??????????????????


Anyone can help me out here


I must admit that I don't exactly know how these things (hydrovac and master cyl) work, since I only have the TM9-801 manual and this doesn't have a nice cut-away view. If you guys have a recommendation of what book I should get ... ???

Is there only 1 bore in the master cyl between the actual cylinder and the container? If there are 2 (1 forward, 1 return), is there a chance that only the forward is open if the linkage is misadjusted? Then that would explain a lot since with every press on the brake, more fluid is pressed in, but none can escape back into the container...

Thanks in advance!
Best regards from Belgium,
Frank

Frank,

If I missed you saying you did this I stand corrected but the first thing I would do is back off the Master Cylinder/Pedal rod adjustment and set that to spec..

For diagnosis you could probably pull the clevis pin and see if there is any preload (for lack of a better term) on the push rod. I believe it should be just about snug...which means that you can install the clevis pin into the clevis easily with the pedal at full rest.

I hope others in the group with more experience will chime in.

On my 1942 Chevy fire truck, which has the same rear brakes, I also had a hanging rear shoe. It was a broken spring. A quick visual did not indicate a break but when I hit it with a screw driver it fell apart.

New Springs every time you do brake work!

Bill

I would replace the brake spring, as they are known to get weak. the wheel cylinder will push it out but the spring won't pull it back enough to release the pressure on the shoes. I believe Dr. deuce had a report on this at one time also. but I have found this was a problem on my jimmy. It's worth a try, as they should be replaced after all these years anyway.

Best thing to do is to pull all wheels and service the entire Hydraulic system and put fresh components on starting with the Master Cylinder, and possible replacement of the other 5 wheelcylinders. This way you can enjoy a safe reliable truck without having to constantly tinker with it. Brake cylinders do wear out.
Joel

I had the same problem with a right back rear wheel and found the problem was the spring. I replaced it and the problem was solved.

Hi all and thanks for your replies!

Well, today I did a quick second test. I made sure that the master cylinder is not under presion from the linkage (the spring that pulls the brake pedal back is actually pretty new and strong and pulls the end of the pedal linkage onto the adjustable limiting bolt). I loosened the screws on the back of the pin that goes into the rubber booth and into the master cyl., and gave this pin a little extra play.

Lifted the right rear wheel, engine off, but after pressing the brake pedal about 5 times, the brake was braking the wheel again. I even tried when I gave the linkage extra play (which is noticed that you have a brake pedal movement for a couple of mm/an inch where nothing happens). Then also the brake gets stuck. Note that all test were done with engine OFF (and without having the truck moved an inch) and off course the pedal got hard after applying the pedal a couple of times.

A couple of you guys mention to renew the spring. It's not exactly clear to me what spring is meant: the one on the master cyl that pulls back the pedal, or the spring that pulls back the brake lining
For this last one: that is strong too because I checked that when I did the wheel cylinder.

Anyway ... I think I will take out the master cylinder this weekend and examine that first ... although that one looks new too, also new rubber booth ...

I must admit that, although the brakes don't work like they should (they work too good ) the guy I bought it from really didn't save on new parts

I'll keep you posted on my findings.

Best regards from Belgium,
Frank

The spring I was talking about is the spring that connects the two brake shoes in the wheel. It could be anything though. That is just what solved my problem and the spring appeared to be strong also, but there is a tremendous amout of pressure placed against the brake shoes that has to be pushed back into the cylinder when the brake pedal is released.

Some of the European Armys converted to Silicone brake Fluid, and when Surplused, the inexperienced owners that would not pay the extra for silicone would add conventional DOT 3 fluid. Your fluid may be Just old with a lot of residue in it, and the cure is to service the entire system by inspecting and replacing all hydraulic cylinders. The truck is old and any maintenance was probably years ago and your symptoms tell me that youe cylinders are gummed up and need replacing. That is my experience with over 50 years around CCKWs and Jeeps and other MVs.
Joel

You should test it with the engine running to find out if the hydrovac is releasing. Sometimes they stick and either release slowly or keep some pressure on the system. Also if the hydrovac was replaced with something else and it has a check valve in it, you will then have 2 check valves in the system which is not good.

The springs are a BIG problem. DON'T ever leave home without at least one and a tool to put it on! Jim Carter Chevrolet has them for the rears and front on the Banjo axles. Same spring cheap insurance at $6.95 as I recall.


Steve AKA Dr Deuce

Frank,

You said in reporting your diagnosis that the front drums were getting warm. Did they not release like the rear or were they just getting warm.

Before messing with the Master I would raise the front end and see if either or both of the front brakes hang up.

Or raise one rear side and see if the rear brakes on both axles are hanging up.

If these are not hanging it is probably not the hydraulics that are causing your problem. I would replace the spring before taking the master apart.

Bill

The only GM regulation Hydrovac with a check Valve that requires the removal of the Check valve from the Master Cyl is the Post WarType #3 Hydrovac Short Fat one Bendix BX 374750, It is asy to ID as it is held together with "J" Bolts that have nut on one end, and are bent into "J" on other end. Type #1 and #2 have rods threaded on each end, and require a check valve in the Master Cyl.
Joel

You indicated that the flex hose to that cylinder was free of obstruction, but it is still a possibility. In my garage business we frequently change hoses for brake problems like this. Fluid under high pressure will flow to the cylinder past a bad spot in the line. This bad spot traps the pressure on the cylinder side when the pedal is released, hence keeping the brake applied. This is a common problem when the #*^hole who did the last maintenance on the brakes used vice grips to pinch off the line so as to not loose fluid. Its worth checking anyway.
Barry

You indicated that the flex hose to that cylinder was free of obstruction, but it is still a possibility. In my garage business we frequently change hoses for brake problems like this. Fluid under high pressure will flow to the cylinder past a bad spot in the line. This bad spot traps the pressure on the cylinder side when the pedal is released, hence keeping the brake applied. This is a common problem when the #*^hole who did the last maintenance on the brakes used vice grips to pinch off the line so as to not loose fluid. Its worth checking anyway.
Barry

hey Frank, How did the brake problem work out? I've been watching this line and would love to know what you found out if anything. I've run into this type of problem what I did was start loosening brake lines till the pressure stopped dropping,if you can understand that.

Hi all. I didn't have time to do any further repairs.
However, today I can do some more repair and I will first check if the brake releases if I open a bleeder screw on another wheel cyl. If yes, then I will have to take a look at the master brake cyl. and the springs that pull the brake shoes back. If not, which means that only the rear rear right brake gets stuck, it looks like the flexhose could be the only possible cause.

I will keep you posted!
By the way: I imported this truck from Holland and I now have it approved for use in Belgium ... I has a LPG (Liquified Petroleum Gas) installation which gave me some problems to get homologated here in Belgium. But it was worth it ... if you think about the gasprices here in Europe ... regular fuel: 1,40 Euro/Liter - LPG: 0,38 Euro/Liter !!

Best regards,
Frank

Hi again from Belgium!

Update on my stuck brake concern: I didn't have much time to work on my truck lately. However, I did one test. Lifted the rear right wheel which was stuck again. Then I bled the rear LEFT wheel cylinder and the right brake got free again. So I concluded that the cause can not be the flexhose! Because, supose, there is an obstruction in the flexhose keeping the wheelcyl in applied position, opening another bleeding valve should make a difference because the rest of the system is pressure free ...

And, in my eyes, it can't be the spring. Because, suppose the spring is faulty and the rest of the brake system is working good, the brake system itself should be without remaining pressure! The brake fluid should flow back into the master cyl fluid tank.

So I'm still going with the idea that there is an obstruction either in the master cyl or in the brake line between master cyl and the T-junction to the both brake lines to the rear wheels.

I'm curious how far my theorethical view on this will bring me ...

I'll keep you informed!

BR
Frank

Hi all !

Well, it seems that the problem with the stuck brake might have been fixed! What I did: I replaced the spring that pulls the brakeshoes back together.

Why I decided to do this first? Well, I always though that the brake system, if not applied, is pressure free. But that doesn't seem to be the case. Also a friend even told me, in order to keep a little pressure on the cups to prevent them from leaking, that the system is under a little pressure.

So I ordered 2 new springs. When I got them, I just knew this would be the solution because these new ones were so strong, I couldn't lengthem them by hand. The one on my truck I took of once I this one was much softer.

So today I fit the new spring and did a testdrive and the brake stays cool. Since it's close before sunset I couldn't do a very long testdrive (old-timer registrated cars aren't allowed to drive in the dark in Belgium) so I'll try do make a longer testdrive tomorrow and check it again. But I've got a good feeling about this.

See ya all later!
Best regards from sunny Belgium,
Frank

Frank,

If 2 springs are weak so are the others.

You should order 4 more and replace them all.

Better to be safe than sorry.

Bill

Hi folks,

Well, last Saturday the weather was beautifull and we took the GMC for a tour ... it ran fine untill ... the brakes got stuck again! We did drive for quite a while with quite some braking before it got stuck.

But now ALL brakes got a little warm/stuck and not only the rear right one. So I guess I'll have to take a look at the master cylinder anyway, because the whole system is under pressure.

So the saga continues ... Paul Harvey would say: 'stand by for news' ...

Can anyone confirm/deny that the brake system is constant under a little pressure (to keep the cups from leaking)????

Greetings from Belgium,
Frank

By the way: I'm selling my M35A2 (lack of space)... if someone from Belgium/Netherlands/Germany is interested, drop me a line !