Just to clarify and reiterate what Tom (aka Gubi) has said, the reservoir is pressurized on a series 4 and is self bleeding. Different beast from the earlier spiders. Check the weep hole on the water pump. Seal internally goes bad and it starts dripping behind the pulley......sometimes when running....sometimes when parked.......not necessarily all the time. And for the four fingered fellows, be sure to check with the engine OFF.
Best Regards,
John M