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Not all will look exactly like this. Yours may differ in detail, but the concept is identical.
Removing the snap ring allows the rear (second circuit) internals to be removed.
See the screw near the middle? That's the stopper screw. It sets the rearwards travel limit of the solid spacer. It's what determines how far back the innards can travel. You can't remove the solid spacer until that screw has been backed out.
This part forms the first circuit.
Inside the plunger (which is part of the second circuit) is the concave surface the pushrod bears against.
This is the second circuit. It nests into the rear of the first.
The rear plug that the snap-ring bears on. If you look reeeeal close, you can see the O-rings on the inside and the outside. Those O-rings are what keeps fluid from leaking into your booster, and are the reason a leaky Master Cylinder shows no fluid loss.
This is what seal wear looks like. See the flat spot where the arrow is pointing?
When new, the rubber seals have a sharp point at their open end. This sharp point has an interference fit in the bore, and thus seals fluid in. When worn, it loses its interference fit and exerts less pressure against the bore wall. Eventually, it exerts so little force that it becomes possible for fluid to squeeze past and allow the pedal to go to the floor.
Given that most premature Master Cylinder failures are due to corrosion stemming from poor maintenance, let's have a peek down the bore of this particular failure.
Well, considering I've changed the fluid every year since the car was new, there is not a single speck of corrosion. The rebuilders are going to get a first-class core when I finally return this one.
When I first poured the fluid out, there was zero sludge or gunk. Nothing.
It's interesting, don't you think? Despite the ubiquity of Metric throughout most of the world, such odd things as pipe threads, road wheels, and Master Cylinders are still measured...in inches. Everywhere.