You can check the brake booster with a hand vacuum pump that you can get from
Wal-mart, etc for around $30 (Mitey Vac I think).  Just plug the pump into
the booster, pump up the vacuum and then step on the peddle.  It should go
down a bit an hold.  If the pressure keeps going down there is a leak.  Also
without the tool you can pump the brakes with the engine off and then turn
the engine on and the peddle should drop a bit and hold.

Also possible that the master cylinder you have now is defective (it happens)
or that some crud left in the old fluid reservoir found its way into the new
master cylinder and trashed it out.

Finally, if you changed the master cylinder before you changed your front
brake pads and then compressed the pistons of the calipers without opening
the bleeds (alot of folks do it this way) then you could have pumped crud up
from the calipers into your master cylinder, also killing it.
Hope this helps.
Ken Wilford
Van-Again
John 3:16