C280 1999 W202 engine 112 (V6) EVAP P0455 code

[ChrisLi]

Looking for help to diagnose and resolve P0455 EVAP error and fuel gauge flashing light:

This C280 is clean many new parts new battery alternator and so on.
The P0455 came on recently. The following has been completed so far but not resolved it:
1. new purge control valve (engine bay)
2. new canister vent valve (in rear left wheel well)
3. pipes from engine to vapor canister checked and can pull and hold vacuum with canister removed.
4. vapor canister removed and can pull and hold vacuum and hold positive pressure in the canister (with
pipes closed off)
5. New gas filler cap.
6. No visible signs of issues with gas filler pipes and pipes from filler side to canister.
7. shaking the vapor canister suggests the charcoal is not as free/loose as I might expect.
8. Measured voltage to canister vent valve (unplugged) - rises to approx 7 Volts (dc) when ignition switch turned to position 2 or engine started - I would expect 12 V ? and indeed the new canister valve appears to remain open (to atmosphere)
9. the repair shop said the new purge control valve (in engine bay) is fine - I checked the voltage here too and it varies between about 7.5 V and 8.5 V (I'd expect 12 V)
10. only other thing to note (not directly related) is some yellowing at end flange of fuel pump (but not true leak).

thank you in advance. Chris.
p.s. I continue to search and read other posts but not yet found one that matches this situation.