1986 F350 4X4

My truck has a vacumm brake booster that I am unsure is working or not. The truck sat for 2 years unused. At the moment I am waiting for my brake line kit to arrive since I had a blowout on a brakeline by the rear axle. Is there any way to test the booster without the rear brakes or does it need to be tested with all brakes operational? Right now when I press the pedal it goes to the floor (obviously) but also sounds like there is a lot of air moving in the pedal area when I do. I have though about pinching off the bad lines and trying it then but also unsure if this would matter.

 

my brake petal sinks to the floor slowly at traffic lights and my idle goes down as this is happening, I know thats one good sign.

 

Pinching off the bad lines is a good idea. I can usually tell that the booster is bad by running the truck then seeing if the brake pedal behaves the same with it on or off, then the booster is probably bad. If it's only bad with it off it's typically a master cylinder. And if your master cylinder is leaking out the back, it is a good idea to replace the booster as the brake fluid eats the rubber in it.

 

I ended up plugging the bad line and the booster tested fine. Thanks for the help.