The booster is a sealed unit. There are no replaceable gaskets. If its leaking, its the internal diaphragm. The only seal there would be on the firewall would be to keep the elements outside of your cab. Nothing to do with the performance of the booster.

Remove the brake booster vacuum line, and plug it with your finger. Did you hear a hiss when you took it off? Does your engine run smooth now that its plugged up? All this and hard brake pedal, sounds like a booster to me. I'm not sure about the swapability of the masters.

88 4R, 350 V8, R150 5 speed Sold <...uot; />
97 4R, stock, daily driver
98 Sienna, kid and dog hauler, wife's ride