Another possibility is an internal ram leak, a$$-u-ming you have hydraulic slides. Bad o-rings or a scored cylinder, which can cut an o-ring, are the main causes for internal cylinder leaks. Valves, though, are usually the culprit, more often than not. Get one little piece of cr@p on the valve seat and you'll have a small leak. When you're dealing with high-pressure systems (3k psi and up) that leak will literally eat away the valve and/or seat over time. Amazing to see the damage a microscopic leak can turn into.
Lyle