although on my friends we have never bothered and it does not seem to affect it in any way maybe under extreme conditions etc the header tank is pre Pressurerised to approx 4 psi from memory
i would just check your radiator hose tailings and just reattach bleed and test drive gently if it gets hot again or blows the hose off again most important you let it cool naturally to prevent any damage small hint with six cyl motors when you bleed them do it on a small incline with the front of the motor at the highest point this allows the air from the rear cylinders to exit more freely the hose tails on the radiator may have just snapped the end off it and still look good check for the corresponding piece left in the rubber hose the radiator will need to be replaced as the plastic left will be brittle as well although it may last for the weekend or until you can get another one then i suppose if all else fail there is a possibility of the blown head gasket which should not be a big deal as long as it gets change immediately and has been left to cool slowly the m30 does not have those awful torque to yield bolts hopefully neither does the M5 this is only a possibility and probably is not the problem

• 91-m5 - for a friend - Top rad hose blew off - no damage - BMW-Test-Pilot 2008-05-15 22:55:16 (76 views)
  • Be certain it's not the upper radiator nipple that failed - dave535i 2008-05-16 10:37:40 (65 views)
    • the m5 cooling sys is pre Pressurerised - explodingbudgie 2008-05-17 19:09:44 (57 views)
      • Was top rad to rubber hose that has broken about a third - BMW-Test-Pilot 2008-05-19 08:37:17 (63 views)
        • yes you can just buy a side tank - explodingbudgie 2008-05-20 02:06:13 (52 views)
        • I haven't heard of being able to order just the plastic end cap - dave535i 2008-05-19 09:14:04 (51 views)
          • Located in not so sunny, cold Sydney - yeah I know it's not as cold as - BMW-Test-Pilot 2008-05-21 09:06:23 (44 views)