I am just about to change out my booster as I am having the same symptoms (hiss when brakes are applied with car running and RPM's rise by about 300). I'm curious why the RPM's rise with this condition. My best guess is that the carbs are being leaned out somehow, when air is seeping past the booster diaphragm. Anyone else care to offer thoughts.

Time to replace your booster.

Are these boosters for the 240z available new from somewhere, or do we go the rebuilt route, and who rebuilds them.

Can they be rebuilt in a home workshop.?

The factory service manual indicates there are (were?) 2 service kits for the booster. One kit contains the wear items (seals). The other kit contains the diaphragm, reaction disk, and a few other pieces. You need both kits to do a "complete" overhaul. I doubt you can get these at a part store. It is most likely a dealer item only.

When opening the booster be careful. There is a rather large spring in there that will push the 2 halves of the canister apart. Other than the spring issue it seems like it should be an easy job.

Beer man, when I wanted to rebuild mine last year, Fastwoman mentioned that the Nissan dealerships not longer stocked those kits, in fact, they had not even heard of the rebuild kit, but I would call courtesy anyway.

Keep in mind Advance Auto has brake boosters for 98 bucks....with a core. I installed one and it is working wonderfully.