Different fluids expand and contract at different rates. Brake fluid expands very little, but it does nonetheless. However, that does not mean your brakes are "blocked," but pedal feel can be effected by hot fluid. You'll also notice that in the master cylinder, you don't fill the fluid to the lid, but allow room for expansion and contraction, just like in the coolant reservoir.
Your oil level can be quite different at different temperatures. I always check the oil with the engine at operating temp (oil, not just coolant) and let the car sit for about 5-10 minutes.
BTW, H2O is the only fluid that expands when frozen. All others contract.
1998 C230 "Black Betty"