V-8 oil pump failures
 

I have taken 6 V 8 oil pumps, short style 1932-48 model 50 pound spring. They all have failed shaft bearing surfaces and failed bushings. I previously installed a new melling and at 5000 miles ,it had the same problem. My Cam is excellent, new drive gear and it all looks good. My question is how can it fail when it is under constant pressure? Should the bushings have a oil passage groove cut in them to allow oil to enter? Has anyone else seen this failure. I have also noticed the same problem in the Model A oil pumps.