Hmmmm.
I've been replacing these seals in 235's since I was just a kid (dang has it been that long- well, no-never-mind) and I'm seeing that others are putting you in the right directions.
Check the rear of the valve cover on both sides and rear with a clean white cloth. Any oil? If not, then it is in the rear main seal.
Back to the rear main- For some reason your mechanic (old school- I presume) felt that the rope seal, although a 57 235 comes with the neoprene, was what was called for (He should have known this). Unless, he saw that the surface where the seal mates had some sort of imperfections on it. These old engines can sit for long periods and the mating surface for the main seal can pit or build up gums and tars, often when the crank is turned this surface doesn't get the attention it needs.
If there are imperfections in the rear main mating area these can, at times, be overcome by using the 'rope seal' because it will expand when the oil gets to it, there-by letting it 'fill in' the entire area.
I know this is being fastidious of me, but, check for tar a gum buildup (or heaven forbid, pitting) on the rear main seal mating area.(this is usually where the problem arises) This can be dressed out with some fine emery cloth (being sure to clean the area properly afterwards)
Then follow this
Tech Tip don't, I repeat- DON'T put sealant under the cap- it won't torque nor seat properly.
If you still are having problems with this, feel free to contact me or just keep posting here.
