Leak vs evaporation?

[mas985]

That's what it sounds like. If you have the type with the removable cover, you can inspect the valve itself.

[PoolDoc]

Evaporation bucket test for swimming pools:

1. Get a clean bucket you can suspend in the pool.
2. Fill it with pool water to within 3 - 5 inches of the top.
3. Suspend it IN the pool, so that the pool keeps the bucket at the SAME temp as the pool water. (can use steps, on some pools)
4. Mark water levels in both bucket and pool.
5. Compare at 24 and 48 hours. If the pool loses much more than the bucket does . . . you have a leak.