cavitation during suctioning

I used to be able to suction my gunite pool via skimmer well with suction plate on top of skimmer succesfully.  Now, even if I turn off drain side, I lose suction fairly quickli and pump starts cavitating.  Pool is 20 years old.  Any sugestions would be greatly appreciated.

Thank You

Good morning. So if your skimmer or suction line pulls well when the vacuum is not connected then you may just have a little too much suction attempting to be pulled through that line when the vacuum is connected. If the pump cavitates when just that suction line is on and the vacuum is not connected then you may actually have a clogged line. You may need to have the main drain(if you have one) slightly turned on to decrease the suction to the suction line for vacuuming if the line is not clogged.