Where in the feed hose do you put the backup valve for a 380 polaris pool cleaner

Should be located one hose length back from cleaner head. Sort of tear drop shaped in-line with the hose. It has a gear inside that changes water flow so the unit will lift off pool bottom every couple minutes in case it gets stuck in a corner.

check inline filter or check to see if hose is collapled sometimes the bend where they go into the polaris

It is likely a coincidence but you may have a restriction of the water downstream from the device. This would cause a higher pressure and might therefore add to the stress on the backup device. Regardless of the cause you still need to new backup valve or a new part to replace the part that split. They are not hard to replace if you can get it apart.

If you have trouble getting it apart you can bring it down to Leslies Pool Supplies and they can put it back in order for just the price of parts.

I hope this helps.

I think it is a coincidence. On the other hand, there could be a restriction downstream causing the pressure to be too high. I would check for this but either way you will need to buy a new one. I hope this helps.

One hose length longer than the longest run

remove the backup valve temporarily and note action. If improved - replace valve. Also check head float for water. There should be none present.

If the wheels are spinning independently you have broken drive belts. The hose swivels are leaking also which could lower the pressure to the head.