Rainbow 320 does not drain when pump off

[engrav]

Good morning
am using Rainbow 320 while raising CYA
is a new one so check valve should be fine

directions say
1. Turn control valve to the closed position. SHUT OFF PUMP.
2. Wait one minute. This will allow water and fumes to drain from feeder.
3. Leave control valve closed and turn on pump. The check valve will prevent water from
entering the feeder.

but #2 seems to not be true, water does not drain

which is a problem cuz when you add tabs the water can splash or overflow and is nasty

contacted Pentair who said
1) relieve pressure by opening valve on the filter; did that and did not fix the problem
next suggestion was
2) unscrew Rainbow cap a bit, let water drain a bit, then screw cap back on
but that loses prime (I guess not a big deal but...)

any ideas why water does not drain as on the directions

searched but did not find "the answer"

by the way: tabs dissolve and feed Ok

thank you