Filter Drains when pump is Turned off

[Rambo15]

I bought a home that came with a 12000 gallon pool. It has a Pentair pump and filtration system.
There are 2 valves that control the skimmer, main drain and vacuum. There is a third valve that controls the two shooting fountains. When the pump goes off the filter drains out and so does the pump basket. The next time the pump turns on it takes some time to get it primed and water flowing back through it again. There are bubbles in the pump basket where the strainer sits.
I cannot run the system on full vacuum because it does not seem to suck in enough water to keep the pump strainer basket full.
Here are the troubleshooting steps that I have done so far:
1. Rebuild pump using gasket kit Pentair 356198
2. Replaced the seal plate, seal plate gasket and shaft seal.
3. Replaced the impeller.
4. Replaced the pool filter gasket.
5. Replaced the pump filter gasket.
6. Applied silicone sealant to all suction side connections.

There is no pool back flow preventer check valve.

After all this, I am still getting air bubbles in the basket and the pool filter and pump basket empties when the pump is turned off.

Gratitude for any and all help and suggestions.
Mark K.

[Denniswiseman]

Just seen this, sorry for the late reply
When the pump basket empties out, it's because the filter has pressurised air in it and the air forces the water back to the skimmer where you should notice it
You may also have a suction sideair leak
Have you silicone grease to the pump basket lid