Water gain and loss with pump on and off respectively

[nicky7]

Bought a house last year with a 30+ year old pool and I've been having a strange problem that the local pool guys have been clueless about. Basically, I lose water if the main drain valve is open and pump is off, but with the pump on there's a water gain. Turning the main drain off provides a constant water level regardless of pump operation.

About the Pool
Gallons: 19-20,000
Type: In-Ground, kidney shaped
Skimmers: 1
Main Drains: 1
Returns: 1

The Skimmer has 1 line to the pump house, the other hole is capped from underneath.

Both skimmer and main drain lines have a shut off valve just before the pump. and the main drain has a hydrostatic valve.

Symptoms

Pump on, skimmer and main drain open:
* Pool gains about 100-160 gallons an hour.
* Will overflow

Pump off, skimmer closed/open and main drain open:
* Pool loses about 80-140 gallons an hour.
* Water is pulled (in reverse) back through the return, through the filter, and into the main drain line where it disappears. I know this because if I open the main drain and close the skimmer, you can feel the suction on the return jet.

Pump on, skimmer closed, main drain open:
* No prime and won't prime. Acts as if clogged.
** Update: It seems to keep it's prime, but when I turn on the pump, the water will flow for a ~4 seconds before you start to see air.

Pump on, skimmer open, main drain closed:
* No water loss or gain.
* Pool filters normally (except main drain is closed)

I've run it for about 6 weeks off just the skimmer without any problem except that I've had to vacuum every 2 or 3 days.

I had tried priming and flushing the main drain by sticking garden house wrapped in a towel into the pump basket, forcing water into the main line (when I thought it was clogged). It would build up quite a bit of pressure, but I couldn't get anything to change. When I prime the main drain line with just a garden house, it will continue to take water (albeit slowly) indefinitely, and I can overflow the basket. If I put the lid back on and turn the pump on with just the main drain open, it will pull some water out before the basket starts filling up with air (seems more of a vacuum than an air leak).

The only thing I can come up with is that the main drain is somehow clogged at the drain and that turning on the pump creates a vacuum so the hydrostatic valve opens up and lets in ground water into the pipe going to the pump, however diving down revealed nothing at a glance. I didn't take the cover off, but I plan on doing so the next time I'm in, perhaps the hydrostatic valve is stuck up, but that doesn't explain why I only lose water through the main drain's line between the pump and the main drain.

Any advice is greatly appreciated, thanks in advance.