Digital BW, The Print

I'm having a strange problem. In the past, when I've had a clog on my 
7500 which would not go away I've replaced a few dampers which have 
solved the problem. I Just had a problem with the Black and Cyan 
positions. The black all of a sudden would get brakes in the pattern 
which would not go away. I ended up printing a black only Purge which 
got lighter and lighter until I could barely see a patern. I put in a 
damper and drew some ink into it with a syringe from the port which 
goes on the head nib. I then put the damper back on and printed a 
nozzle check, NOTHING. I did several cleaning cycles and tried again. 
Still nothing. I then went back in and replaced the damper on the 
Cyan thinking that I had replaced the wrong damper. I fired up the 
printer, did a cleaning cycle and then printed another check. Now the 
black did not show up and the cyan shows very little activity. I 
replaced the damper on the black and tried again. Still no activity 
in the black channel and little in the cyan. I've ordered a couple of 
more dampers and plan to start again. How much Ink does the head 
itself hold in each channel apart from the damper. what is wrong and 
how do I correct it. Please help.