commando brakes dragging

I have a 73 commando that has had a recurring brake problem for the last few months-after driving for a while, once the jeep has set for 10-15 minutes, once i restart it, the brakes are locked down-It will move, and once I drive a short distance, they let go, but it has a very hard pedal.i have replaced the booster,vacum line, and check valve, but it still persists-any ideas?

 

I wonder if the pistons inside the wheel cylinders are stuck/sticking.
when is the last time they were freshened up ?

 

Rubber brake lines can swell with age.

 

Replaced both fronts about a year ago

 

Rubber brake hoses can deteriorate sometimes act like a check valve. It took me forever to figure out why the rear brakes on my cj2 kept lockjing up and the hydraulic brake light switch kept the brake lights on.

 

Also make sure the master cylinder isn't overfilled. Brake fluid can expand with heat and apply the brakes.

 

Try checking the length on the pushrod in the booster. If it's too long it won't allow the piston in the master to retract all the way.

Another thing to check is freeplay in the brake pedal. There should be some free downward movement in the pedal before it starts to apply pressure to the brakes.

x2 on the cylinders and hoses. If they aren't fairly new they should be replaced.

 

Have you put a vacuum gauge on your line going to the booster? Just curious what kind of vacuum you're running to it. It sounds like a booster issue the way you describe it, and with you already changing it, I'd lean towards thinking you have a vacuum issue.