I figured this forum may get more hits. My Buddy has a 1994 F350 5.8L, runs great but wont stop when you try to brake. We've replaced the booster and master cyl but the pedal still sinks straight to the floor barely able to stop the truck. Any ideas on what it could be? We're thinking something with vacuum pressure...

 

If the pedal sinks it must be a leak somewhere in the system. Check all your fittings and look for drips. It could be a slow leak but your pedal will still drop.

Its not a vacuum issue... low vacuum would cause a hard pedal.