Low oil pressure after shim

[pearce trucking]

Look at quickserve and the oil pump section of the service manual. You can compare the length of your regulator spring with nominal. Also verify no beat in on the back of the regulator bore. My first guess is that your regulator spring is tired out. Even if it's at nominal length unsprung, it still is probably tired from years of heat cycles. I installed my new pump and put 2 dimes in the dogbone. Now I'm at 42 psi cold idle and 38psi hot going down the road. Up from mid 20s with old pump and no dimes. My old regulator spring was about .05 or .06 shorter than new one. No other major differences. I think they just wear out. Also there is a tool to compress the spring with and measure torque to compress. I don't have one, but my unscientific hands felt a bit more force to squeeze the new spring. Also, that regulator spring is directly responsible for how much pressure your pump delivers. It would be much less likely that any other part of your pump would be responsible.
The bypass valve would cause dirtier oil quicker, but wouldn't likely reduce pressure noticeably. Basically the oil pressure is set by a feedback loop to the regulator. When output pressure rises, the spring is compressed, thereby allowing the high pressure side to vent back to the inlet side. The more plugged the filter gets, the higher the pump output pressure is, and the more compressed the regulator spring will be, which will open a path for oil to return to the pump intake, lowering the output pressure, which relieves pressure on the regulator spring which increases pump output pressure, and so on. When the filter is plugged, the bypass valve opens to ensure oil flow to the motor. If you have the old style valve that leaks, it causes lower pump output pressure. The new bypass valve is more properly calibrated and also seals better when closed. There are 2 sizes of plug for the new bypass valve. 16.5 mm and 19mm. You should use the same one as you already have. However, measure the depth of the bore to be certain. I tried a 19 and crushed my new valve. The 16.5 was right for me. There is a bulletin that I mentioned before that goes over it in detail.