Bought an 86 f150 that had been sitting for at least 15 years. Got it started and although it idled rough it did idle. Once the motor cranked, the brake pedal depressed. The higher the Rev, the lower the pedal went to the floor. The master cylinder was completely dry. Is the pedal a symptom of a bad booster, lack of fluid in the master cylinder or a vacuum leak? I'm going to change the booster anyway but I'm hoping to use the master cylinder after cleaning it.

 

Fill the reservoir and try to work the brakes. Sounds like your booster was working fine, assisting with brake pedal force. If you're lucky the MC just got dry over the years and a refill and bleed will bring things back. Odds are though that the MC has corroded and the seals are shot. It's what they do over 15 years.

Put some fluid in first though and pump the brake pedal. See if the pedal will stay high and hard after pumping. If it slowly sinks to the floor that's a sign of a bad MC.

You'll probably have to change the MC and keep the booster. Check the inside of the booster for brake fluid. Clean it out if there's some in there. It will eat the diaphragm and ruin the booster.

 

The pedal went to the floor with no pressure, foot wasn't even on it. I've never witnessed that happening. The inside of the MC is corroded pretty bad and I figure the seals are gone. I'm just hoping someone can explain why the pedal sank, just odd and it bothers me that I don't know why.

 

I see now, didn't get it before, I thought your foot was pressing the pedal. Someone who knows the inside of a booster might know. There are valves and stuff in there, controlling vacuum. Does the pedal have an intact spring? The spring should pull the pedal back up.

Sounds like you just need a new booster and MC. Good luck. You probably need new calipers and wheel cylinders too. The reality is all brake parts are usually shot after sitting for many years. Brake fluid absorbs moisture and it does its damage over time.

 

The lines appear to be sound, but you are correct all the parts will be replaced. I bought the truck so my dad and youngest son would have a project. I just dislike not knowing.

 

The pedal responds to the throttle because the booster is bad. It moves enough to notice because the master cylinder is empty/ruined. Don't risk your families lives by skimping on brake parts. A new or reman MC for that truck is probably <$30, but you can upgrade to the (thinner & better) '87-96 booster & Aluminum MC at a junkyard and get OE parts for a few bucks more.