Brake Servo Air Leak

Morning.

I have an air leak from my servo, the brakes have always been pants so its probably always done it. The mechanic noticed it the other day I had it in for a brake pipe replacing and when he gave it a swill down at the end of the day to ensure there was no brake fluid left on bulk head there was a distinct hissing from the servo where the master cylinder mounts. As the mechanic knows I do quite a bit myself when time allows he said for me to have a go and suggested unbolting the master cylinder and resealing where it mounts.

As I have no idea how a servo goes together will it really be this simple or will it only be letting air out because it has failed internally?

Thoughts gratefully received as always.

its letting air in

I should point out the hissing was only noticeable when he poured water on it. Like it would sound if it was hot, but it wasn't it really looked like the air was coming out to me but I may be wrong.

time for some soapy water then and blow some bubbles