help with water flow for a o-ring change

[shirl1128]

so I have been fighting a air sucking problem all last season (live in ny) and thought it was the new cartridge filter we put in but the read about it have to be from skimmer to pump. so I flex sealed everything even though i didn't see any leaks. we have a older pump and so we bought a new one this season and after a day running the filter is under pressure and we are blowing bubbles again well this year i noticed water leaking from the handle of the jandy 3 way valve, and I have learned that there is a o-ring set i can replace all great, buttt........ the hose from bottom main drain has no shut off. im reaching out to see what others here have done or any ideas on how to stop the flow while I change the o-rings out.
we have a 21' round above ground and we both cant stay under to unscrew cover.

 

[Texas Splash]

Having an AGP with a drain, you might have to swim down there, remove the drain cover, and plug it with something like a rubber ball, winterizing plug, or something similar just long enough for you to work on the valve.

 

[shirl1128]

Having an AGP with a drain, you might have to swim down there, remove the drain cover, and plug it with something like a rubber ball, winterizing plug, or something similar just long enough for you to work on the valve.

someone told me that a dishpan and a cement block would slow it enough to work on it. what's your feeling on this ?

 

[Texas Splash]

someone told me that a dishpan and a cement block would slow it enough to work on it. what's your feeling on this ?

I guess all options are on the table. Whatever works.