I recently have a hissing sound coming from the brake pedal area and just replaced the power brake booster and now it still hisses at the brake pedal.

The guy at the part store says that it is the master cylinder doing that.
Is He correct??

any help will be greatly aprecitated
thanks in advance Troy

 

No, it is not the master cylinder. It's the brake booster.

If it's doing it all the time you have a vacuum leak. But, if it's only doing it when you press on the pedal, that's normal.

There is a diaphragm in the booster. When the pedal is released vacuum is balanced on both sides. When you press on the brake pedal a valve in the booster allows air to enter the back side of the booster which then, because the vacuum is still on the other side, helps push the pedal down.

Some hissing is NORMAL.

 

I have changed the master cylinder and the power booster now I have brakes but pedal still goes to the floor

 

Did you bench bleed the master cylinder before you installed it?

How did you bleed the system?

Disc or drum brakes in the back?

 

If it is hissing, it is the booster. Was your pedal real hard when you first heard this? You know when you did have brakes. Before you did the master. When you get it all bled out and have brakes again. If it is hissing still and the pedal is hard and the truck is not stopping correctly you have a bum booster. Best to buy them together, pull the master off the booster and bench bleed the master first. Install the booster and then the master. A little trick we use in the shop is have someone in the truck and pump the pedal up as best they can. Then just put a little pressure on it and crack open the rear line at the master till it squirts past the line nut. Then do the same to the front line. This will cause any air you may have gotten in the master while you were hooking up the lines.
Then bleed the r/r then l/r then r/f then l/f if done right 3 to 5 pumps on the pedal from a little brother/ sister or girlfriend before opening the bleeder. Let it squirt. lock it up and do it over 3 times each wheel or untill no air is coming out. Top off master. test ride and do it again if you have to, But you should be fine. REMEMBER IN MY OPINION The brakes are more important than anything else on the truck.
Dave

 

I think what is going on is a bad brake line, the rubber ones, I recently rebuilt the front end, hubs etc and the guy helping let the calipers hang by the hoses overnight. So this might be my problem. Brakes work good now still hear the hiss out by the brake pedal but heard that this is normal.
Test drove it today and the master cylinder was just a little low, could this be the air in the system doing this or a leak somewhere.
I do not see anywhere by the wheels a leak.
Never buy dorman power bleeders as they do not work, just leak.
I took them all out and brakes are fine now but concerned about the fluid going down just a little in the master cylinder.
I will check it after work tomorrow and if it is low im going to get new brake hoses from amazon.com the are only like 18 dollars a piece. at autozone they are 40 dollars a piece. wow

 

I would try bleeding them again. Also if you have drum brakes on the rear may want to take the wheel and the drum off and make sure the wheel cylinder's are not leaking.

 

I have the same issue, but is it a problem?
My wife noticed a couple days ago that the brake pedal makes a hissing sound every time it is pressed but only when the engine is running. The sound comes from the immediate area around the shaft that goes throught he firewall between the pedal and the booster/master cylinder. But when the engine is off, it hisses only a couple of times and then "pumps up" to a hard pedal. Start the engine again and the first and every time I press the pedal down, it hisses.
What's changed? I recently changed my heater core and found the vacuum line to my heater control had been broken. I repaired that and thats the only thing that I can think of that's changed. The truck stops well and fluid is full, however, I must confess I have a long time symptomof the ABS and Brake dash lights on most of the time.
Any ideas?

 

It's normal for you to hear a hiss with the pedal depressed, that's how the booster works.

 

I know this thread is ancient but thank you 00DakDan, I just picked up an 04 SLT last week and I love it, but hearing that sound from the pedal had me worried.