AIO Rinse issues

[acherry]

Hi everyone, hoping someone can help me diagnose this strange issue.

I converted an older Fleck 5600 AIO system into a Katalox Light AIO. The install went fine, gravel + 1.5 cu ft KL, new piston and new seals/spacers in the valve head. I expected the pH to go up at first, but after 1.5 months it still stayed around 9.3, so I started digging into what’s actually happening during regeneration.

Here’s the core problem:

When the system finishes Air-Draw and enters Rapid Rinse, it initially sends water to drain, but after a few seconds the drain flow collapses to a trickle or stops completely.

If I push the piston manually downward while it’s in the Rapid Rinse position, the drain flow becomes full-force, almost as strong as the Backwash flow. As soon as I let go, it drops back to a trickle.

Additional symptoms and observations:

• Backwash is strong and consistent (full upflow, good lift, strong turbulence).
• The piston makes a sharp “pop” when shifting into Rapid Rinse, as if it’s being pushed up by pressure.
• After disabling air draw and running multiple BW→RR cycles, pH dropped slightly (9.38 → 9.13), so the bed is collapsing somewhat.
• But the Rapid Rinse still only works when I manually push the piston.

At this point I’m unsure whether the issue is:

• A downflow restriction in the valve throat
• A partially rolled or mis-seated seal (new seals on an old valve body)
• Foreign material lodged in the downflow port
• The piston riding incorrectly on the seal land
• Or some remaining air / hydraulic backpressure in the media bed

Any insights would be really appreciated — I’m pretty deep into this one and could use expert eyes.

Thanks!