Hissing Sound That Stops When Brakes Applied.....

If you can hear it in the car more than under hood, I'd guess it's the heater vacuum control. My 89 Cavalier hissed and needed a new control valve (and is cheaper than a brake booster). Underhood and heater vacuum hose diagrams (and MANY links to others) are in this post.
https://www.stangnet.com/mustang-forums/threads/ac-heater-vacuum-lines.872381/

Does hiss continue after engine's turned off? if so, there's a check valve/storage tank (which could be the booster) involved.

I use a piece of soft 3/8" clear flexible tubing like a stethoscope to find vac leaks (could use fuel line or anything), though might be hard to get up under dash to heater control. Listen (by moving head/ear) to various areas and see if it's louder somewhere.

A vacuum gauge might be helpful if you can tie it in with the source - you'll see vacuum when it's hissing, and none (or much less) when it stops. A cheapo is fine ($5-10) and you'll need some T-fittings (check HELP! section).

You could isolate inside car from under hood if you can disconnect/cap the vacuum line that goes into the car (vacuum plugs/caps also in HELP! section, or can use a short piece of vac line with a bolt/screw in it)

I had a bad vac leak in 67 Grand Prix... found by hooking vacuum gauge to manifold vac, then disconnecting/plugging hoses. Ended up being the hidden headlight actuator diaphram. Fixing this gave me about 3" more manifold vac and helped with a rough idle.

Not 100% sure of this but gives a way to trace it. Vacuum would drop when you press on the brakes. I've had booster leaks caused by the master cyliner leaking fluid from the back and rotted the diaphram (not sure if Ford's will do this).