Sorry for not getting back to you sooner, I had to locate and test an older model cart before responding.
When you remove this model cart from your printer, they will (should) remain pressurized. Check that the carts are correctly pressurizing by removing them from your printer, and opening the fill hole plug- you should hear the pressure release. At the same time, make sure there is an o-ring around the fill hole for a tight seal, then reinsert and twist/lock the plug closed.
If you hear the pressure release when opening the fill hole plug, and both carts have the o-ring around the fill hole, then I would like you to test ink flow from the exit valve. To do this, you will need to reinsert the carts into your printer and allow it to re-pressurize them (or, if you have a syringe, inject about 20ml of air into the air inlet point of the carts to pressurize). For the next step, I recommend wearing rubber gloves, and working over a sink: remove the carts in question, and holding one exit valve down over the sink, slowly insert the tip of a straightened paperclip straight into the exit valve to depress the spring and open the exit valve- ink should quickly/easily squirt out. If it doesn't, then ink may not be correctly flowing from the cart, possibly due to particle build up or dried ink in the valve- in which case, I recommend you get new carts. If ink does easily flow from the carts when you do this test, then the carts are working correctly, and the issue is somewhere in your printer- possibly trapped air as you suggest, or particles in the lines, dampers and/or print head channel. In this case, I recommend you try a few regular cleaning cycles, or 1-2 power clean cycles to try bringing the black channel back, though if there's particle build up in the lines or damper, then you likely won't see any change after cleaning. As per the Epson repair manuals, we recommend replacing the dampers every 1-3 years to maintain good ink flow, and get both good quality output and a long life out of your printer. Unfortunately, with the 3800/3880 printers, the dampers are not sold or replaced individually, but as a whole assembly.
Please keep me posted with your results, so I can try to help.
Thanks and best regards~ Dana