Tire Defect Light. I believe now!!

  After a day at the track yesterday I got the TIRE DEFECT light coming home (picture is not of my M5).  The car tracked fine with no pulling or vibration, so I thought it must be a bad tire rotation sensor.  As soon as I got home I took all the tire pressures and the front left was down to 34 psi while all the others were at 39 psi.   Never had this light before in all the years of M5 ownership but it was telling the truth.  Somehow I got a small cut in the tread with a very small leak.  I'm due for new tires anyway and might switch from PS2s to PSSs this time.