Thank you for your reply PoolDoc. I don't think I have any kind of pressure relief fitting. Here's my equipment pad, and here's the top of my sand filter. When I try to turn off the skimmer with the pump running, and open the valve so that water can come from the main drain, the pump loses its prime because it sucks in air, presumably from the hole that's in the MD pipe (which, again, must be at the same level as my returns less @ 4 inches). I can bring a little water in from the MD, but only if about 75% of the intake is from the skimmer.

My main drain intake at the bottom of the pool does run off to the east, which is the side of the pool that the equipment pad is on. Here's a pic of the east side of the pool.

What comes to mind is to start digging just south of those black chairs that you see (that's the east side of the pool (I'm looking from the north)). If I'm lucky, the pipe runs under the concrete deck where the deck is relatively narrow, and the leak would be easily and safely accessible and repairable.

Running a new MD line would involve digging under the pool all the way to the bottom drain hole underneath the pool, no? This would no doubt require a permit, and cost thousands? I've seen solutions which involve running a new liner inside the pipes. Are there companies which sell kits to do this, or is there a DIY pipe liner solution? I saw one company, but apparently they're out of business.

The soil around this pool is very sandy, and so the digging would be easy to do by hand.

Again, thanks for your help with this matter.