knowing what kind of system it is would help. If it is single acting, it has to be the valve, because the cylinders would have to be leaking out on to the ground. If it is a 2 way system, raise the bucket and instead of capping the pressure line, disconnect the return lines from each cylinder and fasten it up rather than leaving the end hanging down. Check for fluid coming back out of the line. Not only will that clarify if cylinder or valve, but will tell you which cylinder.
