All Classified forums require a Site Supporter subscription. It is only $25 per year and you can buy and sell whatever you like on the forum. To become a Site Supporter, go to Your Profile and select "Paid Subscriptions."
These trucks came with combination valves which incorporate a proportioning valve. They are incapable of caring about anything. It is strictly based on pressure. read up on the combination valve in the tech section.
disc/disc uses a proportioning valve. disc/drum uses a combination valve which includes a metering valve. You would not use a standard prop valve in a disc/drum system.