Im back for all your help again. Im more curious then anything. Ive looked on google and found both possible answers. When I push my brake pedal you can hear a hissing sound down by the pedal. When you let off it goes away. Now in the past year and a half ive replaced the master cylinder 2 times. It seems that everything is junk anymore. Well 2 days ago I was driving along fine and went to stop and there was the usual hiss. Then the pedal went to the floor and the hiss got louder. I had to hold the pedal all the way to the floor to stay stopped and she still wanted to move. So I thought another master cylinder because theres no leaks anywhere. I got it today went to put it on and the brake line broke so itll have to wait til tomorrow. SO, my question....Whats the hissing and how do I get it to go away? On google some say powerbooster some say its only the powerbooster if its hard to push which its not. I just hate the noise. It drives me crazy. Thanks in advance. Bill 86 f150 4x4 5.0 Automatic

 

It is your power brake booster.

I had better luck grabbing a ford one from a 93 ford f150. The part store ones do not last.

 

Yes, power brake booster.

If you can get a master cylinder, get a Motorcraft. They last a load longer and are a lot higher quality.

When you replace master cylinders and brake boosters, do them in pairs.

When a master cylinder goes bad you can have brake fluid coating the diaphragm inside the brake booster, and this can help lead to early failure.

Also, the pedal going to floor indicates another bad master cylinder or possibly air in the lines. The master cylinder isn't holding pressure and fluid is going by the seals (provided the lines are bled and your new master cylinder has been bench bled so there's no air to compress). Yes, I too have to replace my booster and master cylinder.

But remember, always do them in pairs.

Also, in my case with a bad booster, I can push my brake pedal and the motor shudders as if it has a vacuum leak (due to the tear in the diaphragm of the brake booster). My pedal is also soft because both of them are bad.

If the booster is bad enough, and the master cylinder is good, your pedal will tighten up like a come-along.

 

When you remove it all, check the firewall for leaked brake fluid. Get it off and if the paint is gone at least use some quick rattle can paint or undercoating.

Few things eat paint as well as dot 3 or 4, leaving a great place for rust. I'd take going TO the floor over through it any day haha. Don't ask how I know.