It's job is to limit hydraulic pressure to the rear brakes. Skidding tires have no traction or steering control and will cause the rear tires to try to pass the front tires. Avoiding that loss of control is the reason rear-wheel-anti-lock brake systems were developed for trucks. The proportioning valve is just one part of the combination valve. It also houses the pressure differential valve and the metering valve. The pressure differential valve turns on the red warning light on the dash when there's a leak in one half of the system. The metering valve prevents fluid flow to front disc brakes until enough pressure has been developed to move the rear shoes out far enough to contact the drums. All of that was designed to work together to build a balanced brake system. Even replacing the combination valve with the wrong one will compromise braking power. While a shrewd lawyer or insurance investigator might not catch the wrong part, you can be sure they'll see a missing part. They love to find even less serious modifications than that because they will convince a jury that you were partly at fault for the crash caused by the other guy running the red light because you were less able to avoid that crash, ... And they will be right.