1985 Dodge D150 Brake Problem

The brakes on my D150 go to the floor when coming to a stop. Weird and hard to explain. Only did it a couple times but I have not driven it much either.

They quickly go back to normal. In the driveway there normal etc….. I see no leaks Master Cylinder has fluid etc….. I checked the power brake valve and I can’t blow in sorta can suck out on it. Makes a loud hissing sound sorta when doing that. I see no leaks around the power brake booster. Any idea what could be causing this and what to do to find out what’s up with it? Thanks