Gord&Fran, thank you for the information about the book.
Joe H, thank you for your experience. I also do not know exactly what the existing valve is designed to do, I just know it has been generically referred to as a proportioning valve.
When evaluating options to change the master cylinder I tried to determine what GM trucks that had front disk and drum rears used but the results did not point to a clear preverence for straight bore or what bore size/sizes were used so I decided to start with what I have. If I can resolve the rear brake lock up issue with the addition of an adjustable proportioning valve I will probably go that route but if I need to change the master to a straight bore it would be better to do that at the same time.
