Have you ever opened one to repair, rebuild or inspect it? I just reexamined the one I have on the bench. First, the hourglass slug only moves a small amount before it contacts the warning switch. Hard stop of movement. So the piston only moves a small mount. The springs keep the pistons against the slug and centered when there is equal pressure on both sides. While the plug can move towards covering the port, the piston diameter is much smaller than the bore and does not seal the ports. It just does not move that far and has too much clearance. It would be like trying to use a master cylinder without the seals on the piston. The 2 Orings just isolate the front and rear circuit inside the distribution block. BUT THEY provide no closing of a port if pressure is lost on one side.
None of the service manual or training manuals talk to what you're saying. Makes sense to do it and modern cars may, but the distribution block on these vintage cars don't. No ABS involved here.
If you don't believe me crack one of the lines loose on the distribution block and push on your pedal. You will have stopping power on the sealed circuit, but you will keep pushing fluid out of your loose fitting until that reservoir on the MC runs dry. It just does not work how you're saying. Sorry.
PS Infact if it worked your way, you could never bleed brakes by using pedal, the piston would close it off. But you can bleed brakes by pumping pedal because the piston can't seal the port.
I have the combination valve on my car now. Did the Aspen to Bbody disc frt. and valve from same car. Stops great and straight.
