I have a '71 F-250, and after I shut off the engine, their is a hissing noise from the brake booster. Is this normal? I think the booster has a vacuum leak.

I was thinking I need to replace the Power Brake Booster check valve.

Would this sort of vacuum leak from the brake booster effect the engine operation in any way? My engine seems to have syptoms of a vacuum leak.

Thanks again for all the help!

 

There should not be any hissing sound or it will effect the engine when running.

Check the vacuum lines to the Power Brake Booster.
Check the Power Brake Booster check valve and check valve mounting rubber grommet.

If the condition of the vacuum line, check valve, & rubber grommet look good. Then the leak is in the diaphram within the Power Brake Booster. There fore the Power Booster will need replacing.

 

It sunds like the diaphram in the booster may be leaking and when you shut the truck off it is bleeding off the vaccum Just a thought. I don't think you can replace the diaphram I believe that you have to replace the booster but I'm not totally sure on that Good Luck.

 

It does indeed sound like your PB booster diaphragm is leaking. Check it first like this:
1. With the engine running, see if you can hear the hissing from the booster before you do anything. You may be able to pinpoint it that way.
2. Remove the plastic adapter from the booster. The engine can be on or off, but it may die when you pull the hose off. If it does, just have someone re-start it while you use your thumb to seal the vacuum intake line. (Be careful here not to break the adapter. It may be necessary to turn the engine off to get it out without breaking it.)
3. With the vacuum line off the booster, and sealed with your thumb, with the engine running, listen for hissing at the adapter.
4. If you hear the hissing from the line or plastic adapter, then you have a leak there, instead of the booster. The booster is probably OK.
5. If you do not hear hissing from the adapter, have someone turn the engine off (with the adapter still removed from the booster). If the hissing has disappeared, then it is probably your booster. (This check ensures that you don't have a vacuum leak in some other lines under the dash.)

Now keep in mind that a booster with a leak does not pose a serious safety hazard. Even if the booster fails catastrophically, you won't lose your brakes. You'll just have to push real hard.

However, comma, that vacuum leak poses a problem with your engine. The vacuum line comes off a vacuum "T" off to the side. If the booster is leaking, it is leaning out the mixture on that intake runner by adding extra air. This extra lean mixture can lead very quickly to burned valves and pistons, so you would be wise to change the booster soon if it is leaking.

And, Hog caller is right. They are not that expensive, they are easy to swap out, and repairing the booster properly or replacing the diaphragm is not a job for most.