1992 Ford Explorer no pedal and loose caliper pins

We had our brakes replaced. They put on pads and shoes, with new pins in the front calipers. The truck keeps loosing pressure in the pedal. They have been bled several times, but the pedal still eases back down, as it is drove. We have more pedal when the truck is not cranked. Upon ignition, it goes to the floor and has to be pumped back up. When we have to stop, we also have to pump the pedal. One day, the pins popped out of the right front caliper. It was making a clunking sound and then popped out. That made the brakes go out completely. The mechanic said it might be the booster, the master cylinder or maybe the wheel cylinders that is making the pedal go down. He has been gracious to go back and work on our truck for nothing, since he did put on the brakes, but I am skitish about putting more parts on it. What do you think it is?