not 100 percent sure on the m50 but on the m30 there is a bleed scew on the thermostat housing so that as you fill the system the air comes out.B4 you start tho make sure the bleed screw hole is not blocked,and it helps if the front of the car is uphill or raised a bit so the air naturally goes to the front of the system.I think you are also supposed to hae the heater on and turned on full so the heater core also is filled and purged.Hope others with m50 experience chime in