I have a FEBCO 765 BFP and 7 zone sprinkler system needs to be winterized.

I hooked up a 21 gallon compressor at 50psi output to one of the test cock on the BFP，shut off the inlet water to the sprinkler system, opened the ball valve on the test cock, then manually turned on the valve one zone at a time, but there is no water blowing out.

Can anyone help me with this problem?