Anyone else had this problem?

I just got done with a customers Honda today, Cylinders all the way around. When I go to drive it down the road, whenever it hits a bump or anything that may push on the cylinder it loses pressure and drops a little. I took the entire front back apart and checked for leaks, NONE. What else could it be, do I need to put a check valve on it or what? I've never seen or heard of anyone having this problem.