'90 F-150, Haven't driven this in almost a year and the sticker is due. Anyway went to pull out of the driveway and the pedal goes almost to the floor, it stops fine once it gets there. I thought about air in the lines, but no leaks, master cylinder is full. Had the front pads replaced last year, rear shoes ckecked (ok). You can hear a hiss(vaccum leak)under the dash driver's side. Mech. said I probably need the brake booster replaced. Would that cause this?

Possibly. However, just because the master cylinder is full, doesn't mean it's still working...That is the most likely culprit...The one in my 86 Chevy did the same thing when the master cylinder failed...

can you pump the brakes and it gets firm ??

My bet would be the master cylinder.

matt

if truck has rear wheel abs, i would suspect the abs dump valve. located under truck on framerail. you can test master cylinder by taking lines loose at master cylinder and taking 2 small nails, like sheetrock nails, insert them into lines so that tha nail heads are facing master cylinder, when you tighten lines, the nails will block flow to the rest of truck and let you know if master cylinder is bypassing internaly if the pedal still is soft. it is an old scool trick that will rule out the master cylinder most times

Pumping the brakes has no effect.

master cylinder internal seals are not working correctly.

the vacuum booster is designed to assist the pedal pressure when you push it... if its easy to push- the booster is doing its job.

master cylinders are cheap and easy to replace. bleeding the air out is the hardest part of the job.

well like Ronnie, I say its the master cly...