Interesting GMax problem: won't fill all the way when water pressure is high

[Ethereal]

I have a Toto toilet with the GMax fill valve and recently it's kept running after flushing, ever so slightly. After replacing the cap and inspecting it while filling I've found the following:

- When filling, water streaming from the top hole in the cap is trickling into the float reservoir
- Once the water level is up to the bottom of the reservoir, the trickling water starts filling it
- The float turns off the fill valve (mostly) before the tank water level reaches the holes in the sides of the reservoir
- If I close the shutoff valve most of the way to reduce the pressure, the problem goes away but the toilet fills slowly

Is this a design issue? I don't see any way this would *not* happen given the construction of the fill valve. Oddly it doesn't happen on another toilet that I have with the same fill valve!

I did fully disassemble and clean the fill valve just for completeness.

 

[Ethereal]

So I replaced the fill valve with a new one -- the design was slightly different but I think there must have been a leak in the old valve, probably in the stem which was causing water to leak into the float reservoir. The new one solved the issue and only cost a few dollars more than the replacement cap I purchased.