Closing, Blowing, and Creation.

I have a main drain plumbed through the skimmer as well. The hole is threaded, so I bought a PVC threaded bushing (2" pipe), followed by a 2" to 1.5" reducer bushing, short length of 1.5" pipe, and a ball valve that ends up in the weir door area. I put an o-ring on the threaded bushing and some silicone grease on the threads, and tightened it in the main drain hole. I used a shop vac to blow the water several feet deep and closed the valve. It works well to create an air lock. I would think the compressor would work good. You could put a PVC cap with a valve stem in it on the assembly I made and you would have a nice air lock set up.

A blower is a high volume, low pressure device and is inherently safe to use blowing out pool water lines. A compressor is a low volume, high pressure device and can damage your piping if not set correctly. Your compressor has the potential of showing you where your weakest piping glue joint is. Unfortunately that could be several feet down, under concrete. I think your better off using a blower.

Unfortunately I didn't know any better when they installed the pool and they plumbed the main drain to the skimmer... The way they took care of it was they fed some backwash hose into the line attached it to a blower and blew it out and quickly pulled and capped it... Then they attached the same blower with back wash hose to my backwash port on the pump, opened the valve and blew out the skimmer and returns from there...

Click to expand...

These days, it would have been better to plumb the MD separately, but at this point it is too late. If your MD is not threaded, then you will have to create something similar as the blue hose will expand and this will work. Regarding your MD, if threaded, then you can easily create an adapter add possibly add a plastic shut off valve. The multi-port will be on recirculate and blow everything out through the skimmer.

Please upload pictures of the skimmer basket and equipment pad, as this would be helpful. It would be best to rotate, save and upload (if necessary). As gtemkin mentions, your compressor may show you where the weakest piping glue joint is, possibly causing problems. I would recommend a Cyclone as it is specially designed for swimming pools (High Volume and low pressure). One small adapter at skimmer and you can clear everything out beside the MD. You can attach a hose with will fit in the skimmer line, blow out the system until the water bubble for 1 minute and quickly plug the skimmer. The Cyclone with adapters, etc. will run around $350.

Pool Parts Online with schematics from Pool Parts Store
has all the adapters you will need. The Cyclone can be purchased anywhere. The online store has great customer service. Send them a message with dimensions, etc. and they will respond back to you. I find that going to HD or Lowes is a total waste of time when it comes to pool needs. Thanks!