I rebuilt two proportioning valves this weekend. I used the seal kit from MSA. I paid close attention to the orientation of the seals during disassembly. Both valves had the exact same seals in them and they were identical to the photos above where seal (3) in BR-8 is fluted / grooved on the flat outside edge and dotted on the cupped inside edge. Seal (5) in BR-8 is exactly like the photos above where it is a little thicker than seal (3) and not dotted or fluted / grooved on the outside or inside surface. For clarity, in the case of both seals, the cupped side is oriented to face the middle of the valve.
I personally was easily able to match up the MSA seals with the equal (but not identical) original seals.
I found disassembly and re-assembly to be fairly quick and easy. I used a small c-clip squeezer (not spreader) (OTC brand, 1120) to squeeze the c-clip and remove from valve. Re-installation is a little more tricky. I use the same c-clip squeezer to squeeze the c-clip and insert it partially into the valve. Then I used a 5/16 deep socket to push / press (gently) the c-clip down until it clicked into position.
To be clear, I don't yet know whether the proportioning valve works as I am still rebuilding the brakes on my 240z. I should know in a week or so.
The valve from my low mile car was very clean. The valve from the car I am rebuilding the brakes for was very full of dirt and sediment....but not corrosion. So, I feel like I accomplished something....assuming the valve works. And, I enjoy rebuilding little parts.
I should also mention that I cleaned up all original seals and, to me, they look new....like they could be reused without concern. Again, I think the key thing I accomplished is cleaning out the valve.