brake pedle going down 1986 K20

[akart]

Ok, Posted this once before but am not clear. Bled the brakes and pedal is good. Start engine and pedal slowly goes down. Stops good and brakes lock up but the pedal slowly go down. Was told that this was the master cylinder but there is no loss of fluid in the MS unit. I am thinking that this may be a hole in the vacuum boost diaphragm. I'll buy what ever parts are necessary, but?? This came about after I installed rebuilt front calipers and pads. Before that pedal was good. Is this a bleeding issue or what?? Thank You for your advice. Some where in Alaska.