Spanks suggestion is logical... but anew engine can show some ridiculous pressures first up.  

An easy solution that is completely reversible is to add a few copper sump plug washers under the oil pressure release cap.  Try 3 or 4 to start with and add or subtract as necessary.  Instead of waiting for things to heat up (to get a true idea pressure) just set it at something like 70 psi cold at idle....

Cheers, Ian

This post is not a cause of your pressure, but this is what could result--> If you have 100+psi pressure, there is a very very good chance that you are overpowering the bypass valve in the filter head, and if not that you are overpowering the built in bypass valve in the oil filter itself.  What this means is that you are pushing unfiltered oil into your engine and bearings grit into your bearings.




Now on to the possible cause: Like others have said-- could be your spring, or your plunger. If you are saying the actual spool / seat is falling out of your motor then THAT could very well be your problem. Is THIS what's falling out? 



It is possible that it is blocking the bypass hole. There needs to be a hole that allows the bypassed oil to drain down into the transmission. It comes out of a hole that is near the rear main cap and blows onto the trans case then ricochets into the spinning crank. If this hole is plugged either at the out port or at the inlet, then the bypass valve and spring won't matter one iota.

How it works is the plunger sits into the seat and the spring holds it there. When the plunger is held strongly closed, ALL of the pressure goes to the filter and then the engine. But the spring is calibrated to allow the plunger to lift off of the seat at pressures above 70psi and that oil that is "leaked" past the plunger gets ejected down through the hole and into the trans. If your Spring is too strong, it won't lift off of the seat and no excess oil pressure will be bled off. If there is a "hangnail" in the barrel of the bypass or if the plunger is a slpppy fit so as to be able to become cocked and wedged, the plunger won't back off enough to bleed off the excessive pressure either. 

My advice is step 1: Remove the plunger and spring entirely and replace the cap and then crank the engine a few times (ignition disabled). Wait a minute. Then remove the cap. If a whole huge load of oil comes out, then the drain-to-sump isn't working right. The "wait" time is to give time for the oil to drain back into the sump through the bypass hole. 

Never in my life have I seen a pressure seat come out on it's own. I've always had to tap them and use a slide hammer or other force to get them out. It has required similar force to reinstall.

I used a stock plunger.  It had some scratches in it when I removed it.  I have a couple new ones to try.  I have not tried the Ball bearing.  I have also tried to shorten the spring as well as use an adjustable relief valve.  Don't know where the oil leak is yet.  I may have to pull the motor.

Dogscarf

I was likewise concerned about high oil pressure numbers immediately following a fresh rebuild. Originally, my idle pressure was 90 psi and 100+ at speed (probably higher—my gauge tops out @ 100). Gradually as my motor settled in, the numbers went down. After 4000k miles I'm getting 65 psi @ idle and 90 @ speed. Still somewhat high compared to a well worn-in motor, but the build is still pretty fresh.

If you decide to do nothing with regard to the pressure regulator, at least keep the oil & filter fresh. Change both at 500 miles if you haven't done so already, and every 1000 miles or so thereafter until the oil pressure settles down and the motor is broken in. Changing oil frequently also gives you the opportunity to wipe clean the magnetic oil drain plug—important on a fresh rebuild. I know some will say that the oil & filter don't need changing that often. Maybe. There are also people that will reuse head gaskets and old antifreeze and spend 20 minutes with emory cloth cleaning a used spark plug. Oil's cheap. My peace of mind is worth the cost of 5 qts. of oil and a new filter any day.

Where's your oil leaking from? Drain plug? Entirely normal. Shift linkage? Common—quick & EZ fix. Someplace else? Maybe your oil pressure is indeed too high and you're blowing past some important seals. Let us know, eh?

Is there a ball bearing behind the spring, or the stock plunger?  The ball bearing, along with an "S" spring is way over kill for oil pressure.  That oil pump is taking SO MUCH power to drive that the engine idle must be set too high!

You can either go back to the standard plunger, install one of the adjustable type, or try chopping a little off the spring...

A missing or nicked seat would leak, resulting in low pressure. Something else is wrong with your build.

Fellow Minioids,  The oil pressure on my 1968 Cooper S is too high.  It is around 100-120 at idle.  It also has an oil leak.  The engine is a fresh rebuild.  I have taken out the pressure relief valve, replaced with another and a new spring.  Same result.  I wonder if the seat is missing or nicked.  I put a small pinhole camera in the hole and hooked it up to my laptop but the resolution is too poor to tell.  Is this a common occurrence after machining that the seat for the plunger falls out of the block?  I don't know what else it could be.

Dogscarf
1967 Cooper S
1968 Cooper S