So have a yanmar VIO 70, hydraulic history, small chip caused the boom spool to get stuck, removed that section and fixed the issue, put in new orings from that section out, well the old orings didn't appreciate being uncompressed so a few smaller leaks appeared. Decided to pull the entire valve stack and give it a good going over. Ordered up new orings from yanmar to reseal everything. Reinstall and refill everything seemed to be working well, also replaced a couple lines that were chafed and added a pin grabber coupler. Ran the machine for 6 or 8 hours doing some light lifting and working, logs onto a sawmill no leaks. Went to working some harder digging where it'd bump up against the limits and after a couple hours it developed a leak that really showed when the dipper hit the end of extension. Drained the tank and opened up the valve to find that the oring looks like it had gone through a wire wheel, it was all eroded with shreds of rubber in the void space around the oring itself. Pulled the entire valve block out again and found several orings that looked the same, rubber was rough and abraded, shredded rubber in the vicinity. Some orings were fine, others at random were all ripped up. No shredded debris around the intact orings. I'm wondering if I got a bad batch from Yanmar? the damage seems strange. The system was primed and started carefully after draining and refilling. The oring grooves are about 1.5 times the diameter of the orings and the rings have a very slight protrusion, about .010 and a thickness measuring about .116. ordered up some .125 diameter from mcmaster maybe a slightly tighter seal. I can get some pictures of the damage if that'll help.
Were the ors oem or from a or kit? Some of the kits are not the correct material for the application. On that chip of metal that stuck the spool , when ive seen this happen 9 out of 10 times its something going on in that cylinder that the spool controls.
So the orings were from Yanmar in official looking packaging. That stuck spool had what I'd say was a 50-60 degree piece of cast iron female thread. Now before I got the machine probably about 500 hours back someone put new pumps in place and that's the only spot I would think to find a thread like that, might have been jammed in a hose before finally wiggling free. I have seen Ford parts arrive that were flat out counterfeit, so anything is possible.
Ive seen orings look like what your talking about and it was to much heat or pressure for the material or they were working or moving about in the groove as the pressure would come and go in that function.
I suppose it could well be fretting under extreme pressure, Yanmars do run high, the old orings show no signs of that but they are just flatrings now.
Really tricky to get a good photograph, but the new rings are just shredded in spots.
Since it is on the outside of the oring, that is usually caused when the oring is being squeezed out of position when the valve stack is being torqued down. Believe it or not, I use a couple of drops of super glue to hold them in position. Grease tends to make that slipping possibility worse. I'm not so sure I wouldn't use a harder durometer oring. Sometimes it is possibly the oring being too thick.