When a pump starts to prime, but won’t fully prime (i.e., you can see air bubbles under the lid), it’s likely that the system has a small leak; and it’s usually on the suction side.
Switch the whole system off, and look for a small drip or squirt of water from the pipes, outlets, sweep hoses, and O-rings on both the suction and return lines.
You may have to switch the system on and off several times to catch the dripping leak, but it’s almost always the cause of this issue.
— Al Mangin, owner, Dependable Pool Service, Temecula, Calif.
