Why is the brake pedal on my Lexus sinking down to the floor?
When brakes are not as responsive as what they should be, or if the brake pedal "sinks" down to the floor, this is a possible indication of a braking system leak. It could be a brake fluid leak, or a brake hose air leak. One obvious sign that there is a brake fluid leak is a small puddle of fluid that is present underneath when the car is parked. Brake fluid is similar in appearance to motor oil, but with a texture that is less "slimy".
The master cylinder is another possible problem. The cup seals or the cylinder bore may be worn. Any time that the brake pedal of a vehicle is fading towards the floor, internal leaking or cup seals being bypassed must be considered. A fading pedal that has no external leak is one of the most common master cylinder symptoms caused by the cup seals. It results in a loss of hydraulic pressure. It is likely that the vehicle will inch ahead at stop lights, as brake pedal fading occurs.