Andy,
If you can hear a hissing then your booster is definitely leaking. To verify you can disconnect and block the vacuum hose from the manifold, start the engine and see if your hissing is gone.
What's often not understood is that a failed booster is actually an inlet manifold leak, just the same as if the inlet manifold gasket were leaking, and you will have a lean mixture as a result. (unless of course someone has richened it to compensate and then when you fix the booster you find that the engine is running rich. Some people may notice an improvement in starting as a result!
I can strip the boosters and assemble them with the original hardware.