pump pressure

I have an inground 21,000 gallon pool with sand filter. i am starting to have pressure problems when vacuuming. it starts off fine and vacuums well, then about half way thru, the pressure gage drops to less than 5 and it won't vacuum anything up. i turn the power off the pump, then start it back and the pressure will be fine, then start to vacuum and the same thing happens again. the pressure is fine any other time and does not drop. could you please advise as what is wrong or what i need to do to correct the problem. thank you.

Only thing I can suggest is it sounds like a suction side issue since either a blockage or an air leak would cause a pressure drop. If it's an air leak the air bubbles will be obvious in the pump basket and bubbles will come out the returns. Is the pressure drop sudden or does it gradually decrease as you vacuum? Just another tidbit of info to help diagnose the problem. Also what type of filter?
Al

i have a sand filter. i am not sure if it is a sudden drop or not. i just get to noticing that the vacuum isn't picking anything up, then i check the pressure gauge and it is down to 5. but i have now noticed that i am hearing air in the vacuum hose, maybe the leak is in the hose and that is causing the problem? please advise. thank you

Vacuum hoses are notorious for leaking and it only takes a pinhole to mess things up. Maybe do a good eyeball inspection for damage and you can try just submerging all of the hose you can to see if the pressure holds.
Al

I once had a similiar problem with vacuming. I had a bad O-ring gasket on the clear top of my pump filter, and it was leaking air. Fixed that and now I get about 9-10psi of vacuming pressure.

thanks for your help!
the hose was the problem

Another easy and cheap fix and a happy ending. Hoses like that I've successfully used vinyl electrical tape if the hose is in generally good condition and got a small cut or something. But if it shows its age then it gets replaced.
Al