That will be the Anti Vac valve... MAYBE...Originally Posted by 1A1E382121243B2C234D0 link=1261003341/0#0 date=1261003341
Hisses as you bring the system up and when pressure has been reached it seals off.. Could be a bit of crap and or the spring or seal is dirty..
Follow up with the supplier first... Could be a clean or replace.. Cheep...
Then again... I could be barking up the wrong tree... [smiley=2vrolijk_08.gif]