Myers M13-N water softener brine tank doesn't fill

I have a Myers M130-N water softener which is about 25 years old but has been working perfectly until a few weeks ago.

Normally, there is about 1' of water in the brine tank between regen cycles. However, after the problem started, the brine tank does not seem to be filling. I can manually add water to the tank (with a hose) and run a regen cycle and it works but the tank still doesn't refill.

I have removed the brine tank and cleaned the sediment out of the bottom and ensured that there is no blockage in the water line from the tank to the softener.

Any suggestions?