'76 K25 brake issue

[76JimmyBlue]

I am having a weird issue with the brakes in the '76. The brakes feel like they should be stiffer, but have not had a problem ever being able to stop the truck. However, occasionally when coming to a stop slowly and dragging the brakes I've had the pedal suddenly go to the floor. It happened one time recently where I was sitting at a stop light and felt the brake pedal suddenly lose pressure. One pump and it was back. Could it be air trapped in the system? The only other idea would be the master cylinder is going bad, it has occasionally wept fluid from between the m/c and the booster. Again this isn't a consistent issue of losing pressure on the pedal. Any help would be greatly appreciated.