In a standard swimming pool setup the pump pulls the water from the pool and pushes it through the filter and then it will return to the pool or other secondary pumps might push it to a pressure cleaner or through a solar system.

The pipe from the skimmer box to your main pump is where the water gets sucked into the pump from the pool. A break in this line will cause your pool to possibly lose water when switched off but it will also suck air and blow bubbles potentially causing your pump to lose prime or your cleaner to stop.

The pipes from the pump to the filter and from the filter back to the pool contain water at pressure being pushed back into the pool. A break in any of these pipes will leak (and not suck air) and you will lose water when the filter is running. A break in the underground pipes will also leak when the filter is switched off.

Most filters will self-bleed the air out through the return jets by blowing bubbles. If air is accumulating in the filter and possibly draining the pump out when you switch it off your pump will be sucking air but you may not have enough back pressure to bleed the air. This can occur if you have an oversized filter (not a bad thing) or a new clean one so there is no dirt to build up back pressure. The primary problem is still that your pump is sucking air and rectifying this will solve both problems.

Note: Most automatic pool cleaners come with a pressure relief valve or speed control. This valve is important as it ensures your pump gets adequate water flow. If over-tightened or not used your pump may struggle to draw enough water and this will cause it to suck more air. With adjustable valves always loosen them out until the cleaner stops then slowly tighten them until the cleaner starts to move. This is where you should leave the valve. You do not need to adjust it again. If the cleaner stops it will usually mean you need to clean the filter.

Before testing always clean your filter. On sand or d.e. filters it is often better to test on recirculate as this gives you a full flow of water (if this solves the problem you need to clean your filter).

If air is your problem work your way from your pool to your pump. Check the pool cleaner hoses for breaks, try with and without the vacuum plate (use hole at bottom) which will identify a broken skimmer box, check any connections, especially rubber connectors in front of the pump for breaks. Remove, re-align and re-tighten all connections on the suction line. Check that the hair and lint basket in the pump is seated properly and that the lid is sealed with an o-ring.

Any connections in the suction line such as chemical feeders can also cause problems. The easiest way to check your pool hose is to lift it segment by segment out of the water while the cleaner and pool is operating. You will hear a hissing noise wherever there is a hole, or split.

If you have not made any improvements on your air problem you may have a broken pipe underground. If you have a major air problem then an underground break would also cause the pool to lose water. If you believe this is the cause plug the hole in the bottom of the skimmer box with anything you can find (a rag or squash ball or expansion plug etc) and leave your filter switched off. Measure the height of the water and leave the pool for a few days before re-measuring (add chlorine as per normal). If the pool does not lose water you have a broken suction line underground.

With this information a pool serviceman will be able to advise you the best way to rectify the problem.