I think you are conceptualizing the theory behind vacuum power.
If the fluid is on both sides of the MC piston that is simply it being ' hydrolocked' and only with regards to initial position recovery.
The force in use here is the power first of the driver's foot.. then possibly amplified by the power booster, then the piston pushing the brake fluid.
Now, if you were thinking about the power booster... you would be closer to correct.
The brake fluid confined by the tube in the MC and pushed by the foot has no place to go except down the lines to the pistons at the brakes pushing on the brake pads....

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1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/diesel-discussion/10414-help-i-need-check-stretch.html

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html

Holy necrothread atmagc

Yeah, if its spongy after driving, but not right away, then like you did, check for a dragging caliper or something causing heat -- those symptoms are the classic boiled brake fluid ones. Brake fluid boils at a lower temp when it has lots of water in it, but if the wheel is hot, its probably dragging or a bearing is failing, causing the brake fluid to boil. The water will just make it easier for it to happen.

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'83 300DTurbo http://badges.fuelly.com/images/smallsig-us/318559.png

Broadband: more lies faster.