Slight hissing is normal on a q system btw... Try greasing the plunger rear bush under the rubber boot attached to the pedal rod and listen to it going silent dramatically.
You have two bushes the outer one gets worn a tiny amount on a q system as we use our brakes a hell of a lot more than manual owners do. The hiss you hear is a little amount of air escaping between the front and rear bushes as you press the pedal and the pressure increases between both, that's all.
The front bush is the important one, the rear one is just to prevent dust etc. But I have yet to see a front bush actually fail or even a servo for that matter. Given their location inside the foot well Instead of the engine bay bulk head, they aren't susceptible to the same wear and corrosion that other cars suffer so the diaphragm remains pliable and healthy.
If the pedal is firm, travel is short and the brakes work as they should, leave it alone... Grease the bush every now and then if the noise bothers you.
Only if the pedal is hard to press and the car will not slow down no matter how much effort on the brake pedal you apply, then the servo/ diaphragm is in need of replacement.
Save your money for something that is actually broken and relax.
Turning off the engine while coasting in neutral in a safe place then trying to stop the car will give you the same symptoms to a broken servo. If there is a huge difference between the brakes with the engine running and when turned, you then know it is absolutely fine.