I recently transplanted an engine into my daughter's '91 240. The engine came out of an '89 240 that had 173k miles on it when my son totaled it. We had bought the '89 when it had 30k miles on it so I know pretty much the whole history of the engine. Oil was always changed at 3000 miles and all maintenance regularly performed. However, it did sit in my garage for 5 years without being run.

After putting it in, it seems to run smooth and pull strong - no real issues. Except, for this oil pressure issue. I didn't notice any problems in normal driving. However, I also put all new brake pads and rotors on the car. I took the car out to break in the new pads/rotors which involves multiple increasing hard braking maneuvers. After one of the hard stops, the oil light came on. I immediately stopped and shut off the engine. Checked the oil, looked for leaks, etc. All seemed ok so I started it up and the oil light went off. All seemed good. So I gingerly drove home.

Concerned that this might be a oil pressure sending unit issue, I went out and purchased an oil pressure gauge to verify. Hooked it all up, started the engine up, oil pressure was fine. Somewhere around 60psi when the engine was cold. Took it for a spin and during normal driving it behaved just as would be expected. However, when I did the hard braking maneuvers, the pressure dropped down to 25 psi and stayed down there. If I shut the engine off and restarted it, the pressure went back to normal at around 50psi.

Any ideas that would explain this behavior? I currently am afraid to let my daughter start driving it until I can sort this out.