The power brake pedal goes to the floor when my truck has been setting for days. After motor start up, a couple presses on the brake pedal and it is firm until parked for a couple more days. The rear wheels do tent to lock up when cold and Engine idles normal cold or warm.

 

Doesn't sound like a booster issue to me. If the booster were bad, you would have a functioning brake pedal, it would just take a lot of force to actuate the brakes. This sounds more like a bad master cylinder, or you have a leaking caliper or wheel cylinder. it could be leaking inside one of the rear brake drums.

 

Thanks for reply.I'll pull the rear drums and check.