O-rings are used everyday in hundreds of configurations to seal hydraulics.
The spools themselves dont have seals. The end caps do. And the end caps shouldnt have much pressure on them. I think you have something else going on.
Any pictures of the suspect area?
No pics available yet but I do agree there is something else going on causing the spool to leak by that much to get at the seals. Possibly the spool is undersized or bore in the valve body a little big.