On an inherited inground concrete pool, perhaps 20 years old.
At random times after backwashing the Hayward sand filter with an old Multiport valve ('SFC' brand if that means anything) so the handle will have been revolved 360 degrees and put into two positions - 6 o'clock for 3 mins, then 10 o'clock for 1 min) the pool water level will decrease by about one and a half inches over 12 hours, regardless of pumping or not. This water loss only occurs maybe one time out of three after the filter has been backwashed. It's really a random thing and I don't think there's a leak in the pool itself, which was relined two years ago. Could an old multipoint valve be the cause of such a serious leak? There is no sign of water in the pumphouse.
I would like to have said the water loss is only when the pump is working but it appears not. This has been going on for about nine months now and over winter there was no problem as the pool didn't need backwashing!
Suggestions desperately needed - thanks.