There is another o-ring seal under the rocker arm assembly. I did the same thing on my Honda just replaced the valve cover seals and oil still was getting into the spark plug tubes.I had to pull the rocker assembly there were 4 more o rings on the other side of that hole.I did a plug valve gasket and tube seal replacement on my Nissan truck a week ago, after discovering the tube seal was letting oil back up into one of the tubes. A wood block and hammer is what was suggested to me, and that