I suspect you rarely if ever use the MK ink.
That happened to me, and the suggestions I heard were:
1. Do a couple of switches between PK and MK to try to clear the switch that controls which black is used,
2. Take it to an Epson service center and have that switched replaced,
3. Find somebody's detailed instructions on the web on how to replace that part yourself, after getting a replacement from Hong Kong, and
4. Upgrade to a new SCP800, keeping the serial Number of your 3800 in case of rebates.
I tried 1 a couple of times, but it only gave me a very short term fix.
I considered 2 and read the instructions for 3, but decided to go with 4, when there were nice rebates.
I am delighted with my P800. My 3800 had given me many years of excellent service and I found someone who was happy to take it away and get it repaired.
Good luck!
Eric