my problem i believe was solved by replacing the master cylinder. i insalled new drums and shoes, then then back started to lock. seemed like all the pressure went to the back.

 

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Paul! It sounds like you have a seized rear parking brake cable. Know doubt you have a typical four parking brake cable system. Front, Center, LH rear, RH rear. It will most likely be one of the rear cables. The grease dries up inside the sheathing and if you get any water inside the cable it will be prone to failure. The parking brake lever that sits vertically attached to your rear brake shoes in your drums will lock your drums up even if you are not operating your parking brake control inside your truck.

Solution:
Block or chock front wheels

Jack truck rear axle up and place axle stands in place

Remove wheels, remove backing plate rubber plugs to adjusters
Back off adjusters (two screw drivers should do it)

Remove your brake drums (Caution do not touch your brake pedal in this test with drums removed)
You will have to tear down the suspect rear drum brake side. If you are getting one side that is hotter after driving then the other or notice more brake dust on the inside of your wheel rim then that would be your suspected side that has the seized parking brake cable. Also look at your thickness of friction material on the shoes edge. The shoe that is worn with a taper 3 or 4 inches on the one shoe would indicate that the cable is fouling up the shoes movement. Check the forward facing shoes. Use a vernier caliper or micrometer if you have one to measure shoe thickness at the top and bottom of each shoe. If you get a big difference in measurement then this is your problem. (example, including metal shoe thickness, top = 0.183, bottom = 0.154)
Tear down only one side leaving one side for reference, refer to your service manual for you exact year, make and model.

At this point you should have the parking brake cable protruding through the backing plate at the bottom around the five o'clock position. Note the return spring that covers the end of the cable where you removed the parking brake lever that is attached to the rear brake shoe. Take a tape measure and measure the length of the spring.

Now push in your "parking brake pedal" inside the truck on the drivers side all the way and release it with the release.

Go back and measure the end of the parking brake cable spring length. If its bad you will see the spring on the cable still compressed and short in length, this is seized. A different measurement will also indicate the spring on the end of the cable can't over come the internal friction in the parking brake cable sheathing to return to a normal position.

Remove and replace bad parking brake cable, follow service manual procedural for this. You can thank me later.

 

Experienced techs and mechanics often overlook or possibly do not know that a master cylinder has air trapped internally and must be bled before installation. A new master cylinder should be "bench bled" before installing it. An old master cylinder needs to be bled if the fluid has been allowed to empty, for what ever reason. Often when doing a brake job the fluid is allowed to all run out. Filling the reservoir only traps air inside the master cylinder. THE BRAKE LINES MUST BE REMOVED FROM THE MASTER CYLINDER and short tubes need to be connected in their place. These tubes must run in a loop back to the fluid in the reservoir. The air must be pushed out and not allowed to enter back into the master cylinder when the pedal is released. The air that is trapped in a master cylinder will not come out during normal brake bleeding. The air bubbles will keep returning back into the master cylinder because the master cylinder is the highest point in the system and cause you constant and continuous headaches. An otherwise perfect brake job will be a waste of time if there is still air in the master cylinder. Air in a master cylinder can get you and your loved ones killed.

 

Perfect answer Notafoadfan, This was exactly my problem for years. Even a brake shop couldnt figure it out.

 

I have a 1994 F250 4X4 and I replaced everything on the brake system except the proportioning valve and the steel brake lines. I even replaced everything again and then again on the rear brake that was locking up but the same issue returned. I thought that the fat shoe was on the front and I thought that maybe the rear brake adjusters had been swapped from right to left. I laid awake a lot of nights thinking about this rear brake dragging and getting hot. I would take it apart and check everything and adjust it until it was not dragging and in a few days it would get hot and drag again. I would stop on long trips pulling a loaded trailer and adjust the brake to stop it from dragging. Finally, one day, I disconnected the new ebrake cable and that fixed the problem. The cable was from a reputable company. It was the correct part number and it was installed correctly. It was causing me $$$$ and a whole lot of grief, sweat and time. I have a stack of brake drums and shoes and wheel cylinders and springs in a pile beside my garage. I went from thinking that I was a fairly good mechanic to realizing that I was just a grease monkey changing out parts. The brake cable was not super tight but it was just tight enough to be a royal pain in my hemorrhoids.

 

On these drum brakes, the shoe with the shorter lining point to front of vehicle, longer pad lining facing to rear.

 

I can almost feel your pain. Did you ask about it? Since I am not a good weekend mechanic, I always cry loudly here and the forum usually solves my problem.