I changed my Combination/Proportioning valve back in 2005 when I did the whole braking system. At that time that "valve" was the old steel one and was quite rusty, so got one from, IIRC, Applied GMC. It was Brass, so I felt I had to check it for the proportioning rubber plug. I did it while installed on the coach. First thing I did was make sure the master cylinder was full and then put the proportioning valve bleed tool for holding to the differential spool valve in place. Then remove the brake line going to the rear wheels, and capped the line off with a inverted flare cap. With fluid trickling out of the combination valve, using a mirror and a flashlight, looked up the valve for the rubber plug and spring. NOT THERE!! So I put it back together and bled the line. Apparently some of the brass ones sold back in the early 2000's did not have that proportioning spring and plug in them. So it is possible that some of you may have the correct brass "combination valve". IMO it is best to check it anyway.