Digital BW, The Print

My 1800 is loaded with MIS MK in the MK<PK<&GO positions,and MIS cleaning fluid in the rest. I only use the 1800 for BW prints.The odd thing is that when I do a nozzle check,which shows a good pattern in all three black positions it ALSO records a perfect BLACK pattern in the R (red) position. After a long print session I think it clears!
The print quality is good, even when showing the black in the R channel.
What is happening ? 
Just re-joined the group, & hope someone can work it out.
Chris.