P600 can't pull ink thru MK line

P600 has been printing with CCP CISS for a couple of years, but was having too frequent cleanings for the blacks and VM, so I decided to do some deep cleaning. Used PiezoFlush in the VM, PK, and MK spots with the CISS carts hanging or on top, and printed many flush pages with QTR and many clean cycles to get PiezoFlush flowing thru all three. Unfortunately, I somehow let the MK cart run dry in my last QTR flush page. I decided to put the regular ink CISS carts back in while trying to get the MK back primed. I read the forum post regarding this and successfully switched back to PK and get a good nozzle check (still pink!) on the PK. After switching back to MK and trying one or two clean cycles, I noticed that no MK ink was flowing from the CISS tank and that there are sections of the MK line that has air in it (makes it easy to see that no ink is flowing). So looking back at the CISS fill/prime instructions, I covered the outlet port with tape and tried to use the suction pull from the top port, but can’t get any ink flowing from the tank thru the line! I then inserted the priming plug and pulled some ink thru the output port, but still no flow thru the line. Is there any other way to try to get ink flowing thru the MK line from tank to cart? Any other thoughts?
Thanks in advance,
Dan