I plugged my main drain using a mask and snorkle and flippers. I'd dive down stay upside down and with flippers be able to stay there long enough to get a few turns on the screws holding the screen on. Once removed I gained access to the threaded port and it was the same size plug as the returns. It took me ten or fifteen dives with rests in between and I made sure I had a "buddy" keeping an eye on me. Having said that I would plug the holes in the skimmer first and wait to do the drain. The fact that yours slowed down tells me it isn't in the drain. My stairs have a few cracks and it was a previous repair that had started leaking again.