Brake Issues-Lock up-Burn up
 

Here is one of those posts that I hope will be good information for those who want to tackle. Her is where I started.

I had three cabs. One of which was a 1969 GMC 3/4 ton 2wd with power brakes. Second the original truck a 1970 Chevy 3/4 4wd with manual brakes. Third a 1970 1/2 ton with manual brakes. All of which were drum brakes. I took all of the parts needed for the conversion to vacuum assisted brakes. I still have all three cabs. I ended using the cab from 1970 1/2 ton because it had the most options and it was an A/C cab. So that puts the brake pedal from a 1970 1/2 ton drum brake. From what I can tell it should not matter. I am going to do some more research on that.

Second phase is the pieces from the 1969 GMC vacuum assist setup. I have replaced the vacuum boost and the master cylinder. I have replaced the brake lines with bendable green lines from NAPA but i am not happy with my work so I am going to buy some pre-bent lines not because the NAPA lines are bad but I don't like the way they turned out. My workmanship and patience.

I grabbed the proportioning valve from the 69 GMC also so we can be consistent. I think. Here is where we start having issues and where we start testing. Oh, the brakes calipers etc are from the original 3/4 ton 4wd. I have rebuilt all of the brakes and checked wheel cylinders.

I was driving the truck yesterday as a test drive and everything was going great. Then the last few miles the truck which has a nice new 383 was having issues keeping speed with the 700r4 in lockup. When I touched the brakes the left locked up. When I pulled into the drive way the driver side front brake was smoking. Therefore the title Lock up-burn up.

I followed one of the searches suggestions and followed this link http://www.classicperform.com/TechBo...hoot.htm#pedal
So without further adieu here is the research and testing that took place. It may not go anywhere but I thought a good account of what took place may help some one else.

This morning I readjusted my brake pedal. However it still seems that the push rod is too short. I have had to adjust it further out than I think it should be to hit my brake light which is adjusted all the way out. I started the vehicle and tested my vacuum and it measured about 17 inches of Hg. Which from the tech article is fine at idle. When I increase the rpms the vacuum increase to overe 20 inches of Hg which is still fine. I am now off to check the brake booster by specifications in the tech write up. My initial thought is because I did not replace or I am not sure what proportioning valve I used this may be the issue.

I hope that this write up helps someone in the future. If you have any comments or questions please let me know someone is reading this.

Re: Brake Issues-Lock up-Burn up
 

I have vacuum assisted drum brakes. I know for a fact that the regular drum brakes master cylinder is different than the vacuum assisted. Also the vacuum assisted is different than a disc/drum setup. The major difference I observed when I upgraded to vacuum assisted drums is that the reservoir is the same for fluid. i.e. same size. The disc/drum setup is larger in the front for the disc and smaller in the rear for the drums. I hope tht will help you some.
Kerry

Re: Brake Issues-Lock up-Burn up
 

I appreciate the comment. I checked and since I have a drum brake set up both the front and rear on the MC is the same size.

So I continued with the the testing of the booster pump. I followed the Test Power Brake Booster

If the pedal feels "hard" while the engine is running, the booster isn't operating correctly. If you suspect the booster is defective, do not attempt to disassemble or repair the power booster. Doing so is unsafe and will void your warranty.

Test 1

With the engine off, pump the brake pedal to remove any residual vacuum in the booster.
Hold pressure on the pedal while you start the engine. When the engine starts, the pedal should drop about a 1/4", this indicates that the booster is working properly. Passed
Test 2

Run the engine a couple of minutes.
Turn the engine off and press the pedal several times slowly. The first pump should be fairly low. The second and third should become slightly firmer. This indicates an airtight booster. Passed
Test 3

Start the engine and press the brake pedal, then stop the engine with the pedal still pressed. If the pedal does not drop after holding the pressure on the pedal for 30 seconds, the booster is airtight. Passed

Inspect the Check Valve

Disconnect the vacuum hose where it connects to the intake manifold. Do not disconnect the vacuum line from the booster. Air should not flow when pressure is applied, but should flow when suction is applied. If air flows in both directions or there is no air flow, the valve needs to be replaced.
Verify Enough Vacuum

Check the operating vacuum pressure when the engine is at normal operating temperature. There should be a minimum of 18 in. of vacuum. Vacuum may be increased by properly tuning the engine, checking for vacuum leaks and blockages in vacuum lines.

So everything in this part checks out. Off to the next phase.

Re: Brake Issues-Lock up-Burn up
 

Ok that pretty well eliminates the MC and booster from causing the brakes to lock up unless you have the wrong rod between the booster and the piston in the mc.

1. Did you rebuild/replace the wheel cylinders?
2. Are the shoes on in the correct position?
3. Are all of the springs and hardware on the shoes in good condition and in the correct position?
4. Are the shoes and drums clean and free of grease and or rust?
5. are the brake lines free of kinks that may restrict the flow of fluid "back" to the mc?

Re: Brake Issues-Lock up-Burn up
 

This little detail throws me a bit...
Where do calipers come in here?

Also, the superfluous information about how many vehicles you started with just confuses things.
Tell us exactly what you have, what you have actually installed, and the problem you are having.

Re: Brake Issues-Lock up-Burn up
 

Something else to be aware off...

Many years ago I had a similar issue. One rear brake would lock up and get the drum extremely hot. Checked everything and finally found out that the rubber brake hose on top of the axle was almost swelled shut on the inside.

The pressure from the booster/master cylinder forces the fluid through, but the spring retraction is not strong enough to allow the return of the fluid.

I checked it with the wheel raised up and truck chocked. Ran the engine slowly and stepped on the brake. I couldn't get it unlocked unless I released the pressure with the bleed valve.

I replaced all of them.

Re: Brake Issues-Lock up-Burn up
 

As I sad I hope this helps someone else out also. So the additional information is just background.

I have tested the proportioning valve and have determined it is working properly with a check light.

So on to a couple of the comments which I do appreciate. I am going to take the wheel off and inspect the brakes etc. If they look good and the brake wheel cylinder is okay then it must be the old rubber brake lines. As I said I going to replace the brake lines but we are not there yet.

Re: Brake Issues-Lock up-Burn up
 

Where do calipers come in here?

They don't I apologize for the miss speak. The brakes and all of it comes from the original 1970 3/4 ton 4x4. I am off to check the wheel cylinders. I have rebuilt the rear but not the front so that may be another issue. 68GMSEE I want to understand that the wheels would lock up because the wheel cylinders were bad? Correct?

Re: Brake Issues-Lock up-Burn up
 

So here is the end of the day. Brake pads are worn the adjuster is all the ay out and it looks as is if I never replaced the wheel cylinder. Hell that was over two years ago. So I plan to do my upgrades to the lines including braided lines and new brakes. I assume it will take care of it. The proportioning valve should be the same for all drum brakes from what I have researched.

Re: Brake Issues-Lock up-Burn up
 

In my case, I had already replaced the wheel cylinders in a previous brake rebuild so I knew it wasn't them. The problem was the rubber brake hose at the rear of the frame from the brake line to the rear axle line. After years of setting the brake fluid swelled the rubber and almost completely shut off the fluid flow. A lot of guys are using the synthetic brake fluid these days for this reason.

Re: Brake Issues-Lock up-Burn up
 

Here is a quick update on this portion. I received my brake lines tonight including the braided lines. I went with stainless steele. for $30 I thought a little bling would be cool. Plus it is a good talking point. I have painted all of the brake parts with P.O.R after sandblasting. I am going to replace the bearings while i am in there. the spring on one of the rubber seals was off so this will be a total rebuild of the front brakes. Since I have already replaced the rear.

Re: Brake Issues-Lock up-Burn up
 

I have received my new lines from inline tube and the front line from the master cylinder to the prop valve does not work it has twop 3/8" threads on it. As far as I am aware all of the prop valvs are the same from 67-70 for drum brakes. Is this correct or was a 3/4 ton 4x4 with power disc brakes different?

Re: Brake Issues-Lock up-Burn up
 

I had the same issue on an 85 chevy, but it was the rubber line connected to the left front caliber. It would allow the brake to set up, but not release...I had never heard of this before so I first replaced the caliber. Still had same issue and discovered it was a swelled brake line.