I have a new sand filter this season, which has been working great until yesterday. I noticed that the pump basket was not completely full, which happens when the pressure increases and it needs backwashed, but the pressure gauge reads pressure of only 1. I also noticed that there are some bubbles coming out of the return jet, so there is air getting in the line somehow. I backwashed and everything is still the same. Pressure of 1, flow is a little better, but not much, bubbles in the jet. Usually backwashing increases the return jet pressure in a huge way. Any suggestions? I can't figure out where air would be entering the line either - tightened the pump basket lid and checked everything, but couldn't find any problems.