Sand Filter and Multi Port Valve Expands and Shrinks When Switched On and Off

Last year my 5 year old sand filter cracked up at the top just below the opening where the muti port valve enters. I figured it cracked because I change the sand and decided to re level the pad which made the fittings to the multi port not fit the greatest. Had to put a little more pressure to get them on than I wanted.

Bought a new one this spring and made sure everything was in line and no side pressure, everything was perfect. Should of been no stress at any connection. Well this one cracked almost in the same spot but opposite side. Really frustrated at this point. Dealer said it was covered under the warranty and that the company said they have been having trouble with a certain batch.

New filter, new sand, and a $50 store credit, fire it up. Now when I run it I notice the filter and valve expand and shrinks when I toggle it on and off. Almost like its going to explode and stops and just runs like normal. It's a waterway.

Dealer came out to see for himself and said had never seen anything like it before and not to run it. Was going to get us the heyward filter.

Anyone know what could be causing this? Pressure is fine, water coming out of the returns, etc. It just moves an inch or 2. Maybe this is what cracked the last one, with the on and off everyday for over a month it finally cracked it. I never noticed it moving because I usually turn it on from inside the garage but notice this one because I was out there by the pool hooking it up.