air getting into pump while vaccuming

I have an 1800 gallon above ground pool. Everything has been running fine, it is 1.5 yrs old. I ran the vacuum last week and it was fine, today I went to clean it and it started sucking in air. Only while vacuuming does it do this. I checked the line to the vacuum, it was fine. I put everything underwater (to check for holes) but it still came up with so much air in the pump that I couldn't vacuum. It almost seems like the air is coming from bottom hole in the pump basket chamber. I don't know what to do, please help!

Try switching which end of the vac hose you attach to the vac head:cool:

I am using manual vacuum...will try....