Main Drain - Can"t run pump off MD

[ellry99]

I opened my pool with a new company today. They could not prime pump off main drain as I usually do when I open pool. We could prime pump off spa line but not main drain. (skimmers were not tried as water level was not up yet)

Any idea of what could be the problem with Main Drain? We look at gate valve to MD and it seemed fine.

Thanks,
elle

In-ground gunite 20x40
sand filter
pool pump
booster pump
gas heater