Pool losing water on filter but not recirculate

Hi. I have an 18x36 inground pool. Had it for about 20 years now. Last fall I had a new liner installed. I was losing water, so I had the lines pressure tested and found that the return lines weren't holding pressure. I bought some fix a leak and ran it through with the filter head set to recirculate. I let it run, vacuuming every few hours, for 48 hours. I was thrilled. The level was staying put. Then I switched to filter, vacuumed and backwashed a few times to get the fix a leak out of the sand filter, filled to the mark on my skimmer and ran on filter cycle. It seems now that it holds water set to recirculate, but not set to filter. I'm not losing water through my backwash. I checked that. The filter and fittings are all watertight. I can't figure out why it would lose water on filter, but not recirculate. It all goes through the same plumbing except for the filter.

Any ideas?

Thanks in advance.

Two immediate thoughts spring to mind, and both involve the multi-valve.
1) There may be a broken partition between chambers.
2) The spider gasket may be defective, damaged, or installed wrong.

There could be other causes but either of those COULD cause the problem.

You could also have a leak somewhere in your sand tank. On RECIRCULATE, the water doesn't flow through the tank. Check the drain valve first--is there water under the tank?

Thanks for your reply. There's no water around the tank. It's bone dry. I took the backwash hose off to make sure it's not leaking through there. I'm trying to do longer checks on each cycle, but we've been getting rain that doesn't help. I think I'm eventually going to have to replace the return fixtures again.