When you say "shows as completely empty instead of solid black" are you referring to the level on the printer's panel or the cartridge itself?
If in the cartridge:
On the 3880 this control/oem chip hack will usually make all the cartridges show full all the time. It requires periodic checks of the ink levels in the carts so you don't run out of ink!
If it's on the control panel:
If you are in Matte Black mode the PK will show white on the control board, and vise versa.
If it is showing white but you are in PK and it's still letting you print, turn the printer off, take out the PK (and check your actual ink levels while you are at it even though this has no barring on the level as shown by the control panel. Put the cart back in and boot up. If still clear/white but the printer is working, don't worry too much about it, but I would buy a backup PK control chip just in case. Chips don't last forever. It's good to have a backup.