The sole purpose of the anti-vacuum valve is to allow air into the boiler when cooling, these valves spend most of their life in the closed position so can and do stick closed even when cooling.
If there is a persistent hiss from the machine the chances are it’s the anti-vacuum valve leaking please read on.
It is good practice to open the steam valve when the machine is cooling to avoid a vacuum. leaving the steam valve open until the machine is switched on and heating only closing when steam starts to be expelled.
There are good reasons for this, please see bad tasting coffee.
Removing the anti-vacuum valve for replacement is not a difficult job but the correct size spanner will be needed, a sharp tap on the side of the spanner will shock the joint and it should release easily, if the valve is in the top of the boiler a slow pull can kink the boiler.
Most anti-vacuum valves are not expensive and to avoid problems should be changed on the annual service, new seals are available however the body can and does corrode making it impossible to get a good seal so a new valve saves problems.
When fitting a new valve use a thread sealer and the correct copper washer.
For part numbers Anti Vacuum valves
0403 - 1509 – 1937 – 3124 – 3433 – 5119 – 7230 – 8692 – 8693 – 15379 – 18819 – 29242 – 29062 – 26992 - 24753