Leaking sand filter

[bevans]

I have an in ground pool approximately 25000 gallons. We recently, this week bought new sand filter. It is waterway top mount 26 inch. Completed installation on Sunday. After filtering and backwashing the first day, when we have attempted to turn back to filter after backwash or waste there is a leak at the top of the sand filter. We have taken this apart make sure all the o rings are in good shape and that the top is on secure. There is still a leak.
I have lubed the o ring, the inner seal and made sure all threads were lined up. I can turn pump on to waste and there is no leak. I then turned it to back wash. You can actually see the pressure build up at the top. It pushes top mount up and then starts pouring out the sides.
Again took everything down. Cleared away any debris that could possibly be there were top mount attaches to filter and secured everything in place. I first turned to waste. No leak. Then to back wash for few minutes. Still no leak. To rinse and again pours out the sides.
I don't know if it could be a defective multi port valve or what is going on.

I'm at loss at what else do. Been pool owner 15 years and never had this issue. Especially being this is brand new.

Thanks for your help.

Bethany