Main Drain clog

I have a clog in my main drain that I believe started after I added (apprently too much at one time) CYA to a skimmer. My skimmer has two holes with one going to the main drain and the other going directly to the pump. My main drains also have a direct line to the pump and are on a common line with my vacuum port. I have attempted to use a drain jet from the pump forcing water back to the main drain, but my vacuum port vents the pressurized water. Is there a way to effectively stop this from happening short of capping off the vacuum port? Also, have others ever experienced this situation? Can adding too much dry chemical clog a suction side line?

I've never seen flaked cyanuric acid set up as a hard clog, but I have seen powdered CYA do that. Not sure how it could get in your main drain, though.

It was powdered CYA and I put about 2 to 3 pounds in the skimmer. A significant amount settled in the conduit that runs from the skimmer to the main drain. Prior to this, I could set my pump to main drain without cavitation, now my pressure drops to near zero with severe cavitation if I valve it 100% to main drain. As far as capping my vacuum port, I was considering pressing a tennis ball in the port to force the water to travel to the main drain plumbing. Any thoughts?

I once stopped up a line like that. So, I took some 1/4" tubing, and hooked up to the pressure side and then snaked it down till it reached the clog. It took a couple of weeks, but I eventually dissolved the clog.