Odd noise

Hey, still wet behind the ears. Working on a M35A2C 1969 Jeep Kaiser Multifuel. I've had her since August and had to park her in December when 1 or 4 wheel cylinders blew. I've rebuilt them, regreased bearings, and installed remote master. I turned her on, built up air to 120. Turned her off to begin bleeding. While trying to get pedal to come back up, still working on that, I noticed a hissing noise. I traced it down to fuel tank. When I press the brake pedal when the truck is off I hear a hissing in the fuel tank. If I pump a whole lot the tank will build pressure. I'm not sure if it does it when its running, can't hear much of anything that close to exhust. I knew the tank is not vented and I always released pressure after I drove it, but i have no clue on this, or even how to search on it. Thank you in advanced.

The tank should be vented. Some trucks the vent ties into the brake vent and opens up by the air cleaner. I think there is/was a mod to change this.

Mine doesn't. Large amount of pressure after each trip. Air spews out. So I'm looking at a venting issue. Would that be why I hear hissing when I hit mu brakes in my tank? Thanks for the quick reply.

Look in the T.M. for your truck, There will be a schematic for the air system including all the venting that is fitted for fording purposes.
You can disconnect the fuel tank vent line coming in and re route it. The vent coming out can also be re positioned so that no pressurizing occurs.

Thank you so very much. Ill break into my manual my next day off and correct it.

As We never go deep water plodding, the vent lines from the slave and brake master cylinders are disconnected. And the others are routed to the intakes near the air cleaners.