I respectfully disagree and I think you're splitting hairs. Whereas expansion of the fluid may or may not be the problem, the fluid level would increase when hot. From a practical standpoint, moisture and air are components of brake fluid. PV=nRT or if you want to get really picky (P+n2a/V2) x (V-nb) = nRT - both proven beyond your opinion. Do you check your trans fluid cold or hot and why? Whether that expansion is technically due to pressure from gases produced, contaminants - whatever - the fluid still increases in volume. As a matter of fact, volume is often a measure of (invisible) gases displacing (visible) fluids. Whether royaii may actually be witnessing expansion of the actual pure fluid or not - the volume will still increase as seen in the master cylinder - which is the situation he is seeking an explanation for. I didn't think the specific physics were required in my posts as this is MercedesShop and NOT PhysicsShop. "Very bad brake fluid contamination" is unlikely as the initial post stated that the fluid was just changed. If he has a dragging brake, it will heat that caliper enough to show the increase in fluid in the master cylinder as the inherent gases expand and increase the volume. I agree with you that this is the most likely explanation, however if you do NOT believe the fluid level will increase as a result of heat I do NOT know how you can support your own seemingly contradictory conclusion.
PS. While I welcome any opposing views, I do NOT appreciate the yelling. I'm just trying to help out here as I have received a lot of help here am actively trying to give back.
1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick