You can replace the valve.
I had it done, but not sure how to do it.
I had a diver change it - $ 225.00.
After that you need to monitor the water level to make sure you don't have any other plumbing leaks.
Hydrostatic value usually looses more water / day than 1-2 inches.
Do you have air in the pump filter basket? If so you may have suction leak. If not you may have return leak.
You can shut lines off one at a time to see if it makes a difference.
You can alo pressure test the lines - See Anderson Swimming Pool Leak website.