A somewhat common problem that occurs with dry fire hydrants that use a metal adaptor on a PVC elbow is cracking of the plastic near the set screw holding the adaptor in place. This generally occurs when an over-zealous driver/operator beats on the suction hose fitting with a heavy-duty mallet thus causing the metal adaptor to try and move and then cracking the plastic at the set screw. The only real repair is the total replacement of the suction head. To diagnose the issue, simply hook up some suction hose and send water back into the pipe and look for a leak at the suction head; the leak generally manifests as a regular drip or stream of water. A small water leak is a big air leak.