i have the same problem today.
i've had my pool for 10 years and never had this happen.
today i turned on my filter for the first time this season. all was running well. no air bubbles anywhere and the filter strainer basket was filled to the top with water.
i connected my vacuum hose to the vacuum head and submerged the length of hose with water eliminating all (99%) of air as usual. I place the skimmer end of the hose, still submerged, through the skimmer opening and stick it with in the hole, same as been doing for 10 years. suddenly the strainer basket fills with air and the pump starts cavitating. this NEVER happened before. i let it run figuring it would eventually even out and fill with water. no such luck. i change vacuum hoses and try again, same thing happens. i gave up on vacuuming for a while, the filter and strainer basket returns to normal. i get a bottle of soapy water and spray every connection, clamp, etc, even spray all hoses and slide my hand up and down each length of hose to ensure everything is soaped up. i see no bubbles, i hear no 'sucking air sound'. i reconnect the vacuum and the basket fills with air and pump cavitates again. now i repeat the soapy water procedure while the vacuum hose it attached, but I still find no soap bubbles and can't find a water leak or air leak. i buy a BRAND NEW vacuum hose. problem not solved. my filter hoses on the suction side are 1 or 2 years old. i've replaced the hose on the return side a few years ago too. any hints on what to do?