I was doing my weekly chase down the vacuum leaks exercise and I finally found an actual hiss. It was coming from the brake booster. Or that area anyway. I sprayed it all down with explosive whatnot and could not get a reaction from the engine. Is there something inside of a brake booster that can leak air?

I could also hear it for a few seconds after I shut the truck off.

 

If it's hissing from inside , then it's not holding vaccum very good, but not so bad to loose power brakes as a result, something could be ruptured inside, (most likely the rubber diaphragm, or a seal) that's why when you shut engine off, it hisses until all vaccum is lost,,
How is your brakes,? If it's ok right now, then pretty soon it will get worse when booster stops holding any vaccum at all,

 

My brakes have been bad since I bought it in 1999. When I press the brakes down then have to press them again quickly, there is resistance and I have almost no boost (as compared with my ot:her vehicles... Ford, Nissan, Mercedes). Always been that way.

 

If brake booster doesn't work pedal will feel very hard under foot & it will be almost impossible to stop the truck,,,
If you are pressing pedal twice to make it work most likely air in system, or master cylinder rubber packing is worn & as you press pedal down some fluid escapes,

 

Sorry, Bom, I should be more clear. For years, the brakes go to the floor before I start the truck if it sits overnight and then take about a minute to develop useful boost so, if I let it sit, running, for a little then back down the driveway I have decent boost when I get to the bottom. Long driveway. Then I go one house to the stop sign and stop. There is a curve. I start up then see a car and hit the brakes, and I have the stiff pedal no brake thing.

That's just an example but one I live every day.

It has been like this for years. I'd given up in the brakes and was looking for vacuum leaks (I have a transient P0171,174 issue that rarely trips the SES light but shows up as pending quite often) today when I heard the hiss inside the booster.

So I wonder how big a deal it is to pull and rebuild the booster? I'm game for anything but this is an unknown... I don't even know where to start.

I've done a lot on my truck over sixteen years but nothing with the booster and I really don't know how they work or how big a deal it is to pull one and replace whatever might be leaking air.

 

Rebuilding booster can be easy if it's a simple diaphragm replacement, but I have no idea how you can get the part for it,,,
I don't even have an idea how much is a new booster compared to a good used one,,,
Try pricing them out, I would go with a new one if not very expensive, given that brakes are important for a heavy truck like Disco,,,
If you want to rebuild yours, I suggest opening it to see exactly what's wrong, you might be able to find parts for it that are shared with other boosters,