brakes-hissing noise

My previous '84 F150 did the same thing. I always assumed it was a vaccum leak.

Right; the booster itself is leaking. You can hear it because it's leaking inside the cab.

It's the brake booster leaking. I put one on my 84 F250 for the same reason. I think it was about $100 plus the core. Fairly simple to install yourself.

Gary

thanks for the replies.

My first thought was the vacuum host going up to the booster (I believe) that sits behind the master cylinder might be leaking, but I couldn't locate any leaks in the hose. I pulled out the small plastic part that fits into the booster and connects to the vacuum hose.

Brake fluid levels are down a little, but not too much.

Questions -what is the small plastic part that connect to vacuum hose. Could it be what is causing the hissing noise? -Occasionally, I have smelled , from inside of vehicle, what I believe is brake fluid. I can change out the master cylinder if necessary. Could the master cylinder be a contributor to the 'hissing' problem? -the master cylinder is a 'lifetime' rebuilt part (Autozone), and has been changed several times since I purchased it. Would it be better to install a brand new master cylinder to keep from having to replace the rebuilt one every few (5-6) years??

I replaced the rear brake cylinders, both sides, several months ago.

Thanks for any help.

Oh yes. A vacuum leak causes idle problems