today while driving ( 1990 F150 ) i went to turn. So naturally I went for my brake pedal and it had very very little pressure. I mashed her to the floor and got her to slow down to make the turn. I then stopped and got out to see if there were any visable leaks, nothing jumped in and pumped the pedel looked again nothing. Shut the truck off and pumed the pedel and all I could hear was air rushing into the cab. I started her up and drove back roads to my grandparents with absolutely no brakes. Checked it over no fluid leak, MC is full, but still get the hissing in the cab. So I took the gravel back roads home about 14 miles and checked the brakes often still nothing. I'm guessing its the booster but most of the online resources say its probably the hydraulics somewhere. Also if it is the booster I have a spare but how hard are they to change out??

 

Dump valve on the brake pedal assembly.