Sand Filter - Alive?

This is my 1st pool and I constructed it myself with a lot of tips from this forum. Now I need to understand, if what I have is operating normally.

What I have:

18'x36' IG Vinyl pool - 2nd season
Individual 1.5" Tigerflex lines to 1 drain; 1 skimmer; and 2 returns
Pump located 12 feet from pool and all piped 2" PVC in pump house.
30" Sand Filter PENTAIR TAGELUS TA100D w/2in 6-way valve
1hp Sta-Rite Max-Glass II
Aqua-rite Chlorine generator
Polaris 360 pool cleaner without booster

When I turn the pump on/off, I notice that my fiberglass sand filter tank visibly shrinks and expands. It's as if it were breathing! When turning the pump on, I can see the top of the filter rise about 1/4" to 1/2". I know that the pressure from the pump causes this but, I'm worried about my sand filter exploding if this is not normal.

My pressure (even just after backwashing and all strainers clean) sits at about 25psi. With the pressure side cleaner attached it jumps to 30 to 35 psi. When I remove the eyeballs from the 2 returns to unrestrict the flow as much as possible, the pressure drops to about 15 to 20 psi.

Are my operating presssures OK? Is a breathing sand filter normal?

Thanks in advance for your feedback.

The filter tank breathing is normal but at 35 psi it's going to expand more than normal. Somewhere on the filter or in its literature should be a max pressure rating. 30-35 psi sounds high and the fact the pressure drops dramatically when you remove the eyeballs says they are severely restricting your flow. Since you really want to continue to use them to direct water flow I'd bore them out as much as you can. If you have access to a drill press it's easy with a cheap drum sanding kit.
Hope this helps.
Al

The retuen eyeball holes now are about 3/4" each. Enlarging the return eyeball holes sounds like a great idea! It's so simple and obvious, but completely overlooked by myself.

The only problem is that my pressure-side cleaner works best when the free return eyeball is as small as possible.

I like your suggestion so I will use larger holed eyeballs when I'm not cleaning. My sand filter will be less stressed and it will probably save a bit in electricity at the reduce load.

Thanks!