[notevenclose]

Bore seal between vacuum assist and master cylinder falure? Can they pressure check the master cylinder?

You say you are not losing any fluid but gaining air in the system...does the level in the reservoir increase? If not, you are losing fluid and it is being replaced by air. This could be a cracked line, loose fitting, etc. If I'm not mistaken, you can see brakefluid with a blacklight in the dark...you could try that to trace for a leak. Otherwise, you could have them add a dye to the fluid and replace the fluid later.

Maybe try to heat the fluid up by dragging the brakes and see if you can find a leak?

Just throwin stuff out there...