Digital BW, The Print

Wondering if anyone's had this happen to them: I switched from a PK  
to a previously used MK (matte black) cart on my 2400, and all the  
dark areas on my next print came out cyan. Eventually, I removed the  
MK cart, put it back in, and it now prints correctly. I'm assuming a  
sticky valve. I keep the semi-used MK and PK carts in a ziploc bag  
with the air pressed out while not in use.

Harry