Good Afternoon, I have a Hitachi EX120-3 excavator with a hydraulic leak coming through the o-ring behind the front anti shock valve cover located on the top side of the swing brake valve mounted on the top of the swing drive. The round shock valve cover is fastened with 6 socket head 5mm bolts. O-ring located behind cover has been replaced with a JD OEM o-ring. Hydraulic fluid is leaking from under the cover and through the bolt holes when STARTING to swing to the left and again when braking AFTER swinging to the right. Pressure spikes from either of these two operations causes oil to squirt out from the bolt holes. It does not leak when swinging to the right or breaking after swinging to the left. There is another anti shock valve cover located in parallel on the back of the swing motor and that one is NOT leaking. I removed the brake valve cover assembly and Spotchecked/Magnaflux the head and cover for cracks. No cracks. There are two swing relief valves located on the swing device and they were cleaned/serviced and fo und to be working as required. I swapped the placement of the valves with no change. Any help on what could be causing this to leak would be most helpful.
The only thing I can see that will cause the leak is the 0 ring. It could have pinched or is the incorrect thickness. Like you I'm guessing. A old bloke who used do my hydraulic work always said " check and recheck the last thing done ".