I have a 2004 RX300 AWD Auto which has done 60000 miles. Recently, when at a traffic light stop, the brake pedal went gradually to the floor with the usual foot pressure while waiting for traffic light to go green. This was after I have driven about 150 miles. At the same time, I could feel mechanical movements in the rear brakes like the brake calipers opening up and some noise. However, despite this, the brakes worked normally when I continued to drive. This happens only when I have driven some 30 or 40 miles and I think if I have done some hard braking like from 50 mph to stop. The brake pedal is normal when it has not been driven for several hours. My mechanic thinks it is the brake master cylinder orings leaking when the system is hot. Appreciate if anyone has similar experience or any idea of what the problem could be to write to me at chongleekwong@gmail.com