I'm work on a 2006 ford e350 with a 6.0 diesel and the brake pedal slowly sink to the floor . Not the master cylinder as it was replaced and if I disconnect vacuum pump and plug the port there is no more sinking . Just wanted to verify it's bad with smoke test and smoke was pouring out what possible could be a vent but I'm not too familiar with vacuum pumps . My question I guess is is a vacuum pump suppose to vent out or is that what my problem is here ?
What do you mean by "vacuum pump"? Do you mean the power brake booster? Brake master cylinders can still be bad out of the box, new or remanufactured. I went through 3 of them myself last year before I got one that didn't leak internally and cause a sinking pedal. They don't always leak externally.
Disconnecting the vacuum line from the booster and the pedal being firm afterwards doesn't necessarily mean that the master cylinder is good. That booster puts A LOT more pressure on the system than your foot can. It will make weak seals internal to the BMC obvious. All of my failed master cylinders seemed fine while I bled the lines with the engine off. As soon as I applied brakes with the engine running, the pedal slowly sank. That car is also 43 years old. It has no ABS. The ABS module can leak Internally and cause the same problem.
Scotty shows how to diagnose a master cylinder here.
