Some boosters will have the piston connected to the pushrod. This model doesn't.

I think the problem is twofold. The booster slave cylinder piston check valve is stuck or plugged restricting all but a little flow through it. And there is some air in the brake lines. The spongeyness generally associated with air in the lines isn't noticed because the master cylinder is just pushing against the nearly plugged slave piston. Repeated brake applications gets enough fluid past the piston to compress the air. The air expanding provides the volume to cause the pedal rise. The brakes will lock until enough fluid makes its way back through the slave piston.

Before exchanging the old booster I'd take the slave cylinder apart just to find out what's been going on. Might just be that the piston ports are plugged and can be cleaned out. Usual disassembly procedure is to remove the big plug on the end of the cylinder then disassemble the diaphragm end and use the pushrod to push the piston out. Might not be necessary to do that. Since piston is likely plugged it may be possible to blow it out with air applied at the master cylinder connection. Might work to take it apart on the truck. Take off the big plug and squirt the piston out using the master cylinder. Could be messy.

Bad thought here. This condition may have been causing brakes to drag. Front brakes may have been loose because linings are worn.

Craig

 

I pumped the brakes with the truck off. No sound from air filter behind seat. Pedal is firm and does not pump up. Start truck and pedal eases down 1/2 inch or so. Repeated pumps bring pedal to top of travel. I did the whole drill about 5 or 6 times.

I am assuming the slave cylinder is located in the booster? I know, I know. What happens when you "assume" things. This is sounding more and more like the culprit.

 

I couldn't find a cylinder using the term slave for the booster, but I did find this and it is inside the booster:

TEAA-2425-B .. Brake Booster Hydraulic Cylinder