What could cause excessive oil pressure in the oil tank of an 1988 evo, plug blows out of oil tank, when loose plug is left in oil weeps from tank and caskets have started to leak. I have changed the relief vavle and springs as well as breather valve

Check your crankcase breather hose. On the '88 model, it originally came from way down low behing the oil pump up to the backside of the air filter plate. If the bike was purchased used, the original owner may have rerouted the hose in order to avoid oil getting into the air filter. Find the crankcase breather hose and check to see if it has been plugged by something or has become crimped.

I'm confused by the term "breather valve". I don't know of a breather valve on the engine. If you mean the "breather gear" in the cam chest, this is good as the originals were made of plastic. These would break a tooth off the gear and quit turning. Usually the broken tooth would lock up the oil pump and indicate a problem in the area. However I have seen on occasions that the oil pump was not locked up by the broken tooth and the engine would build up too much pressure inside. I've seen the pushrod tubes start to leak because of this. If I have misunderstood and you have not pulled the "nosecone" on your engine to check the breather gear, that could be your problem as well.

But, do not pull the nose cone unless you know what you're doing. I've fixed several bikes because the owner went to check the breather gear and didn't know what all was involved in doing this. The nosecone would come off about a quarter inch and hang up, would come the rest of the way off and wouldn't go back on either. If you need specific instructions on how to take the "nosecone" off, repost and I'll tell you how to do it. You'll need a pair of tappet block pins to do the job.

Good Luck
Steve