Sorry, can't help with the problem diagnosis, but replacing the plunger with a steel ball is seen as a way to avoid the plunger getting stuck in the bore due to a bit of swarf.
For what it's worth, here's the method DocMini described in another post.
1. Make sure the spring is straight, or get a new Cooper S one.
2. Pinch a CV ball (9/16") or go buy a new one from a bearing shop.
3. Grind a chamfer inside the best end of the spring so it sits on the ball better. I use a pointed stone in the die grinder. It's not essential if you can't do it.
4. Cut a piece of smooth 5/16" or 8.0m steel rod 40mm long, deburr the ends and put it inside the spring. This stops the spring bending and pushing the ball sideways off the seat (with instant oil pressure drop).
5. Remove the old relief valve plunger. I use a 10mm dynabolt, put it inside the plunger and then do the nut up a bit. Then you have a handle to pull the sucker out with. But sometimes they come out easy with your finger (if the right size).
6. Assemble it all into the valve, refit the cap, start engine and check pressure. If it's too high, trim 2mm or more off the outer end of the spring.
I run mine @ 70-75psi hot from 2500rpm up. Cold, it's ~90psi...
