Dear Mark, Greetings from the Bronx, Check to see if there is a
blockage in the tubing leading to the valve block.  Also check to see
if the finger that opens the tube is not blocked or not opening far
enough.  Also try cleaning the bleed in the valve block.  Check also
if the nipple in the valve block is loose, if it is, make sure it is
not pushed in too far - as it will effect the operation of the bleed.

Hope this helps,
Sincerely,
D. F. Rowe
Bronx, New York