Digital BW, The Print

I'm seeing something odd with one of my
MIS transucent ez-fill 2200 carts.

Yesterday, after a week or 10 days of not
printing, I got a bad nozzle check in one position. 
When a cleaning didn't help, I pulled the cart and 
noticed a bubble around the output port (these 
see-thru carts are great.) I used a syringe with a 
bottom-fill adapter to draw the bubble out, 
instantly got a good nozzle check, and made a print. 

Problem solved, I thought.

This morning--same problem with the same cart. Bubble 
over the output port. Extracted it and immediately 
got a good nozzle check.

Advice? Is it possible this cart is defective 
somehow, and that air is leaking in at the 
bottom? 

-david