C4 master cylinder operation question

69427

The Artist formerly known as Turbo84
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Guys, I'm a bit confused about how the C4 m/c works. I see a couple different places selling a replacement spring to modify the rear pressure bias, but looking at an exploded diagram, that side unit in the casting just looks like a combination/warning switch setup to me. What am I not seeing here?
 
The C4 has a prop valve w/ warning switch in it on the front. It's behind the big plug that screws in from the front and the switch terminal is on there too. Combo valve is for disc drum (combo as in prop & hold off valve)
 
The C4 has a prop valve w/ warning switch in it on the front. It's behind the big plug that screws in from the front and the switch terminal is on there too. Combo valve is for disc drum (combo as in prop & hold off valve)

I tossed the switch valve out of my '72 here, some years ago....and note the same sort of valve, looking identical by memory, is in the camper, which is disc/drum....so question is....what is that prop & hold valve??

is there any significant difference??

:ghost:
 
The C4 has a prop valve w/ warning switch in it on the front. It's behind the big plug that screws in from the front and the switch terminal is on there too. Combo valve is for disc drum (combo as in prop & hold off valve)

Well, the part behind the big plug is called the combination piston, and looks to do the same warning switch function as the C3 unit I have. That function makes sense, looking at the diagram. I'm just not understanding how this piston does any proportioning, given that the diagram seems to indicate that the combination piston sees the same pressure on each end. The spring behind the plug looks to put a preset on the piston, but hydraulically, I'm not following what's going on here.
 
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