hondo

I bought a house recently that came with a 15-year old spa built into the ground (probably a custom spa) and surrounded by brick; the bottom of the spa is probably a little below ground level. The pumps are about 25 feet away, and the pipes to the pumps are under brick as well. There are three capped air tubes near the spa (two on one side of the spa, and next to each other; and one on another side). The skimmer and drain pipes connect under the brick as well. The spa has a blower, which seems to work fine. Problems---I don't have a lot of suction from the skimmer. And I have a lot of air bubbles in the return line. I have checked the accessible pipe connections for leaks (most of it is underground), replaced the filter pump, and blown air through the pump line. Still the same problems. Not sure what's going on, or how to fix it. And since I'm a novice, I don't even understand what those air tubes are supposed to be doing. Thanks in advance for any input.