My pool has an attached spa with a spillover. Usually, the water level in the spa is several inches higher than the water in the pool, but for the last few days, the water level has dropped steadily during the evening after the pump has stopped running for the day. It seems like the water is draining back into the pool from the spa until the water level between the two bodies of water are at the same level.

Why has this started happening? Or, more importantly, why didn't this happen before? Is there a one-way valve that is supposed to prevent it?

Yes it is called a check valve and sometimes they will get old and not seal as well. If it is the type with the clear lid that has screws, then you can inspect/repair them. If not and you determine that is the cause, you may need to replace it.

I see a check valve with a clear window on one of the pressure side lines. How can I tell if it isn't working? And, if I open it up to fix it, won't I have a geyser on my hands?

Is the check valve below the water line? If it is, there should be isolation valves to prevent the water from gushing out. Anyway here is what you need to do:

Turn pump off
Close isolation valves if you have them
Drain filter by opening the air bleeder valve and remove pump lid
Unscrew the valve cover
Inspect the valve flapper for any tears and/or cuts
If fine, relube and reinstall; if damaged, replace

Also, if you have pool spa valves, if the valve does not seal, the spa can drain that way as well. The valves can be inspected the same way as the check valve.

The check valve is above the water line. Not sure what I was thinking.

The valve looks like it's in decent shape, so hopefully it just needs to be lubed. If not, can I swap the flapper from an identical valve or will I need to completely replace it?

You can swap the flapper ... At least I know you can if it's a pentair valve. You just slide the SS pin that the valve rotates on out. I used a small awl. One side will be abraded to hold the pin - tap it out from the other end. I assume the same is possible with jandy brand valves bu I don't know for sure

Not only can you swap the flapper but you can also just purchase the inside of the valve.

http://www.poolcenter.com/parts_valves_ ... 1_7512.htm

Thanks for the advice. Looks like I have a project for this weekend.

This happens to me occasionally when switching from spa mode back to pool mode. One of my valve actuators (on the suction side) doesn't close off the spa drain completely, so it's about 15% spa drain and 85% main drain. The actuator on the other side works reliably, so it's 100% pool returns. The result: water from the spa slowly drops, the pool level rises, and the overflow drains into the back alley.

Switching back to spa mode and then pool mode a few times helps.