I am trying to blow out my sprinkler lines and I put together an adapter to connect the air compressor to the faucet pictured below. The air compressor was charged to 140PSI, and then I turned on a zone and opened the air compressor valve to 40PSI (into the faucet). But as far as I could tell, nothing was happening as far as water getting discharged from the system. It appeared that all of the air was flowing out of the pressure vacuum breaker.
The past couple of years I have hired someone to blow out the sprinkler lines and I know they attach to that faucet. I also know that when they are clearing the water from the system the sprinkler heads popup and the water discharges from them. So what am I doing wrong?
EDIT (2010-12-28):
I am not sure why I was not able to blow out the lines through the pressure vacuum breaker (I imagine it is not sealing correctly as said by @gregmac is his answer) so I ended up installing a new valve beyond the PVB which is where I attached the air compressor.
Best Answer
My guess would be that there is something wrong with the backflow preventer.
Most likely, the air inlet valve is not sealing properly. When pressurized from the side marked "normal flow" (which is where your tap is), it should open the check valve, and also force the air inlet valve shut.
It's probably a rubber gasket, and might be worn out and need to be replaced, or maybe something is physically holding it open. Try pushing on it to see if you can get it to close all the way. Some silicon grease around the gasket will help it form a good seal and prolong the life of the gasket.