Hi, Folks.
Here is a link to a video clip on Youtube which gives some information about maintenance and repairs to Cat CCU's. It refers specifically to rear-mounted DD CCU's but the major components of the front-mounted single-drum CCU's are very much the same so it may also prove useful for those units.

http://www.youtube.com/watch?v=yG7Yu_qKJRY

(Note: This is posted here as a temporary measure until somebody with more knowledge than I have of the workings of this technical section starts a section for 'Attachments'.)

Hope this helps.