Check The Drain Plugs:
Very often the drain plugs on the pump housing will start to allow the pump to suck in air. You can correct this by removing the drain plugs from the pump housing and either applying some Teflon tape to the tapered threads, replacing the o-ring or replacing the rubber washer. Depending on what type and brand of pump you have will determine what you will be doing to the pump housing drain plug. (Teflon tape, replacing o-ring or rubber washer)
Check The Suction Side Pump Union:
On most swimming pool plumbing unions that are made from PVC you will find an o-ring inside that can fall out, break, crack and sometimes even flatten from old age and use. You will first want to open the union on the suction side of the pump(usually front of the pump) and inspect the o-ring inside. If the o-ring, is flattened, cracked, broken or even missing you will want to replace it other wise your pump will suck in air and never have the correct prime. You also will want to make sure that your pump union is tight, so make sure to use a large sized pair of channel locks to tighten the union, but be careful not to over-tighten the union because you could crack it.
Check The Pump Cover Gasket Or O-ring:
You always will want to check the pump cover gasket to one, make sure it is on the pump cover and two to make sure that it is not broken, cracked, or flattened to a point it is not sealing the pump cover when its tightened down. You can generally replace a pump cover gasket in a matter of seconds if the one on the pump you are working on needs to be replaced.
Check The Mechanical Pump Seal:
In between the volute of the pump and the motor is something called a mechanical pump seal. If this seal goes bad the pump can draw in a small bit of air. Most of the time the seal will have to be in really bad shape. If your pump is leaking between the electric motor and the volute / pump housing chances are you have a bad seal in your pump.
Check The Eye Of The Impeller:
If the eye of your pump impeller is clogged up with dirt or debris that could be causing your pump to not run at full prime. You could easily mistake this for a pump that is sucking in air, when really the impeller is just clogged. You will have to remove the pump housing and the diffuser in order to clean the impeller.
Using the above pool pump troubleshooting tips above you should be able to remove all the air from your swimming pool pump. If you have checked everything above and still have air inside you pump, filter or swimming pool return lines you will have to check the swimming pool suction line, skimmers, main drain and also pump suction valves for air leaks.