1989 Suburban - Sluggish Brakes

[SoCalSlaughter]

Hello all, truck is a 89' Suburban V2500 4X4. My grandfather drives the truck mostly. He came to me and said the brake pedal was hard to press and the truck wouldn't slow down. I test drove the truck and confirmed the truck was having a hard time stopping. Once the brake pedal gets 3/4 the way to the floor it seems to bottom out and loose its brakes force. Example, if you where to press on the brakes harder, the braking force increases. This is not the case, you can stand on the brake pedal and the brake force is the same once the pedal reaches about 3/4. So if your going say 10 mph and stand on the brakes the truck will slowly roll to a stop, not lock up the tires at all. So far the master has been replaced (Says my grandfather, about a year ago) I've bleed each corner of the truck twice to make sure there is no air, did a hand vacuum pump test on the booster and there is no leaks. The truck is ok driving around town at slow speeds, but going down a hill is a problem. There is no worry of locking up the brakes in their current state.