Good call Innuendo, however the pedal at no time should sink all the way to the floor if that's what is happening. I would also check to make sure the master cylinder isn't riding on the booster push rod at rest. It sounds like bypassing of fluid past the master cylinder seals. If your mechanic has checked this properly, then do a couple of easy basic checks you can do yourself. I would start with the following.

1) Start the engine with your foot of the brake pedal and get full vacuum up then turn the engine off. Press down the brake pedal with steady even pressure and hold. If the booster is leaking and after a few seconds the pedal will start to push back, this indicates a vacuum leak. If the pedal slowly sinks half way and feels like its touching metal to metal you have a problem on one hydraulic brake circuit. If it sinks all the way to the floor you have a problem with both circuits.

2) Place a pair of multi grips on the flexible brake hoses. Clamp the hose just enough to stop fluid transfer. Clamp the two front and the one rear at the diff then retest. The pedal should be rock hard and won't go down. If it does then the problem is in the master cylinder or proportioning valve.
If the pedal is rock hard and doesn't sink after a minute then the problem is most likely sticking front caliper slides. If you disconnect one of the three clamps at a time, retest and if the problem reoccurs than you will have isolated the problem to that brake or front or rear. It's possible the caliper pistons, uneven disc rotors wear or pads can also course the same problem but sticking caliper slides are more likely.

Hope this helps.