I have also been thinking about this, I've had some people tell me that the proportioning valve actually holds some pressure on the rear circuit for drum brakes. I haven't got that far yet but, if its true then it would be a bad thing for rear disc's. I was also thinking that since you have two lines going to the front brakes and only one to the rear that would mean less volume to the rear, but that might not matter if the rears take less volume to squeeze. So if i amy add to your question do you need a specific PP valve for rear discs?