so.. the RR brake line blows a leak. replaced both rear lines. bled both rear and front lines. pedal nice and firm. turn on truck and pedal no longer firm!
re-bleed and same: pedal firm, turn on truck, pedal not firm. kinda smooshy.
any ideas what it could be? vacuum leak on booster?
thanks.
1999 F250 SD 7.3L 4x4

 

First, with a diesel you have a hydroboost, not a vacuum booster.

With engine off it takes a lot of force to generate 300 hydraulic psi. With engine running, it's easy to generate 1600 psi. If there is any air that got up into the ABS valving due to the line loss, the difference in pressure will highlight an air bubble compression.

Another thing that often happens when bleeding brakes is if you pushed down all the way tot he floor with an older brake system that has not had periodic flushes, the master cylinder cups can get caught on some oxidation in the back of the master cylinder bore and tear.

But if the pedal stays high during the engine off testing, it's most likely air. If you have 4WABS the controller can be a bear to bleed out if you don't have the factory scanners. If you have RABS, the RABS controller on the frame under the drivers seat has a bleeder on it. But at the age of this truck and if you are up in the salted road country, all bets are off that you won't find a big rust ball instead of a controller.