I have a 1995 F250 XL 4X4. It is on an isolated Island and sits for long periods. When I start using it with only light pressure on the barke it will drop to the floor on its own. I need to use the other foot to pull the pedal up. After some driving the condition improves No fluid leakage cylinder full.

 

Do the brakes lock and hold , or no braking when this happens ?

 

And Welcome to the site !

 

B

brakes lock then pedal gradually comes up

 

Welcome! Sounds like the master cylinder piston is sticking...

 

Disconnect the booster vac line and plug it with a bolt or stick cut to fit , yes redneck . See what it does then .
Possibly a bad brake booster .

 

Could be that too . Rule out the booster also .

 

I would also say master cylinder. But since it is on an island that means water (humidity) water & brake fluid don't mix in fact brake fluid attracts water. So you may need to flush the entire system & install new fluid. Look and see how much rust is inside & around the M/C.