brake question on 1990 ranger fr

hey all, i'm new to these forums but could really use some opinions on this. i'm driving a 1990 ranger xlt with the 4 liter v6 and automatic transmission. this morning i started getting problems with the brakes. they work fine at first, but if i stop and stay stopped, the brakes start giving out after about 15 seconds and the truck starts rolling forward. i had to throw it into park real quick a couple of times to keep from rolling into the car in front of me. i've never done any work on brakes before and would appreciate some input. thanks Are there any brake fluid puddles under the truck? If not I suspect the master cylinder leaking internally. Look where the booster and master cylinder connect for trails of fluid. Its a good sign its bad (there doesnt have to be any for it to be bad) i haven't seen any fluid puddles, and the fluid in the tank is full.