I ran across a junk clutch master when packing up my shop, so I took it apart and looked at how it works. It's actually a fairly good design, more complicated than I first had imagined.
First, some pictures:
Piston at rest position. Note the position of the lower seal in between the two holes in the bottom of the reservoir socket:
Piston fully compressed. Notice that the upper seal has stopped before the large hole. Also note, the piston has a rod extending out the end that acts as a positive stop for down stroke:
A close up of the two holes. The rear hole is large, the front hole is very small. The lower seal passes over the small hole but never the large hole:
You'll also note that at the lower end of the piston assembly above that there are small holes, keep that in mind.
Here's a drawing showing the piston in both the up and down positions. Note the seal relationship to the holes:
Basically how this operates is that the space between the two seals is alway filled with brake fluid that is open to the reservior via the large hole. The pressure seal is just before the small hole at rest. When the pedal is pushed the seal goes past the small hole and at that point the system develops pressure. When retracting if the pressure falls to zero, say because the fluid contracted in the slave due to temperature changes, before the piston gets back past the small hole then fluid is sucked through the small holes mentioned earlier, past the seal and into the main bore. When at rest fluid is free to flow through the small hole, though it doesn't flow fast because of the small size. This explains why it's slow to bleed. Because there are two seals it's possible for the pressure seal to go bad and lose clutch function without the rear seal leaking into the passenger compartment.
After studying this, I am of the opinion that it is a fairly robust system and determined that my theory about how air is getting into my master is invalid. Also, if GM had simply connected the fluid line to the bottom of the cylinder instead of the top the clutch system would become self-bleeding as all air would work its way up and out through the holes.
Some measurements:
Piston diameter: 0.697"
Cylinder bore: 0.701" (give or take, it was fairly rusty)
Stroke: ~1.5" maximum due to the piston stop, but may be less because of carpet/pedal interference.
JazzMan
[This message has been edited by JazzMan (edited 11-23-2005).]
