One of your rams seals are by-passing internally, so technically you've got a leak but it's internal in the ram therefore you can't see it, it's just creeping back to tank. Two things you can do to check this out:
1 - blank off the feed pipes to one ram completely and take it out of the system all together, try operating it on one ram (don't use any load as you could bend the ram) and see if it holds - if it does then your other ram is the one that's by-passing.
2 - Crowd your bucked forward to the max and then remove the feed pipe to the front of the ram, again operate the forward crowd, if you have any oil coming out of the ram then you've got by-pass on the piston seals.
Hope this helps, good luck
Eddie
Eddie, May 2009