"LPB disc brakes"? It seems like my block will work on my disc/drum setup, is that what you are saying? Definitely do want to drop the money unless I need to.
Used proportion valves are like used master cylinders ( even more so with cast iron body ) in that while they lay open to atmosphere with residual brake fluid inside... Moisture is drawn in and rust grows.
If my memory serves, someone did find a online vendor offering "rebuild" parts or kits. A factory service manual states that these valves "are not serviceable".
If I ever had put the disc brake setup under my car, I would have went ahead and purchased a new proportion valve, and probably the adjustable type.