Anyone have experience with Hydroboost???

[Motor City GMC]

When i test drove this truck the first time before i bought it the brake pedal was very soft and went to the floor, ot would stop the truck barley but when it did it would do so by locking the back brakes, so me and the owner bled the brakes out and added some fluid and the pedal came back and all was good, i drove it home a hour and a half.
The truck sat for the next few weeks while i worked on it and the next time i drove it i still had a nice stiff brake pedal but the rear brakes was doing the same thing again very tou hy and locking up when braking.
Fluid is full, the only thing i am questioning is that i know i have a small power steering leak, could this be the problem?
What do ya think????