Hydraulic Brakes

Hydraulic brake systems use fluid pressure to transmit force from your foot on the pedal to the calipers and wheel cylinders at all four corners. Here is the physics behind why it works: Pascal's Law states that pressure applied to an enclosed fluid transmits equally in all directions throughout that fluid. A small force on a small piston in the master cylinder becomes a much larger force on the larger pistons inside the calipers. That mechanical advantage is what makes moderate pedal effort generate thousands of pounds of clamping force.

Apply firm, steady pressure to the brake pedal and hold it. If the pedal slowly sinks toward the floor while you hold it — and there is no visible external leak — the master cylinder has an internal bypass fault. The piston seals are worn and fluid is passing around them internally instead of holding pressure. The result feels like a slow leak you cannot find anywhere on the vehicle. Replace the master cylinder.

Brake fluid is hygroscopic — it absorbs moisture from the air continuously over time. As moisture content increases, the boiling point drops. Fresh DOT 3 fluid boils at a minimum of 401 degrees Fahrenheit. That same fluid at 3.7% water content can boil at as low as 284 degrees — a drop of over 100 degrees. During hard braking, fluid temperatures can reach those levels. Boiling brake fluid creates vapor pockets in the lines. Vapor compresses. Fluid does not. The pedal goes to the floor at the worst possible moment.