Chevrolet Repair: 91 Chevy pickup brakes, rear shoes, chevy pickup

Question

Well, I'm not sure, but we did flush the system with new fluid, and the pedal is firm and doesn't leak down when the engine is not running.  Is there a check valve on the booster and where would it be located?
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-----Question-----
I have a 91 half ton extended cab Chevy pickup.  The brake pedal will go to the floor when engine is running but not when the engine is dead.  My mechanic has installed three rebuilt master cylinders.  He has bench bled them each time.  He has installed new pads on front and adjusted up the rear shoes.  He has installed another used electronic device located on the fender.  He has installed a new equalizer valve.  He has installed 2 new vacumn boosters. he has isolated the front from the rear brakes and had no success.  There are no leaks on slave cylinders or anywhere else.  The truck has fantastic stopping power, but if waiting at a stop light with brakes held on, the pedal will gradually go to the floor.  If you kill the engine, the pedal stays firm and high.  Vacumn source is good.  Mechanic gave up after consulting with two other buddies working at different Cheverolet houses.  My mechanic is not employed by Chevrolet, but runs an independant repair shop.  What else could be wrong with this system?  Alan
-----Answer-----
Have you tried gravity bleeding?
I believe there is a pattern in which must be followed when bleeding the brakes.

Answer
The check valve is a valve so that when your engine quits running, the vacuum that is in the booster doesnt escape, so the driver can still (with a little more resistance) stop the car.
Have you recently replaced the vacuum hose going to the booster?