Started the other day. When I apply brakes I get a high pitched harmonic sound coming from front of truck. NOT A GRINDING NOISE like when pads are worn thru (just in case I did pull tire and pads / still lots of meat left on pads). This noise will continued even after truck comes to a complete stop; as long as I have my foot on brake petal. Buddy who drove truck said he thinks its a vacuum leak. I used the propane tank testing method briefly around some of the vac lines, nothing so far. Few months back Another buddy who has a 1990 F 150 with similar equipment said after driving my F 150 that brake pedal takes a lot of pressure considering these are power brakes. Ideas are appreciated.

 

Maybe the brake booster,pull the vacuum line out of the booster and try it.
(while it is parked)

 

Yup, sounds like the diaphram on the brake booster is shot. Giving you no power brakes and a whistling noise from the vacuum leak when you step on the pedal.

 

Is this an easy repair? whats involved and how much does that part cost?