HI,
I HAVE AN EXPANCTION TANK PROBLEM;WHEN DRIVEING AND PUTTING ON THE CLIMATE CONTROL,OR EVEN THE HEATING, WITHIN THE FIRST MILES THE TANK LIGHT COMES ON - ON CHECKING WATER HAS GONE,I HAVE BEEN SERCHING MB FORUM PAGES AND REALIZE TO BLEED IT MAY CURE THE PROBLEM,ALSO MB,DEALER SAY GET THE HEAD GASKETTS CHANGED;"AS THER IS EXUST GASSES IN THE WATER"AND IT STARTED WHEN I GOT A GARRAGE TO CHANGE THE WATER PUMP;ML 500+2002=62000miles

 

to bleed the system, keep the cap off, and fill the overflow tank to the proper fill mark. with the engine off your going to sqeeze and massage the upper rad hose and then the lower rad hose. this will help to push out air in the cooling system. the level in the reservoir will drop down as air is being released from the cooling system. you can start the engine and turn on the defroster or heater to FULL heat. rev or hold the engine until the thermostat opens, youll see the engine temp raise up..probably a little further than the NORMAL operating temp is. then the thermostat should open and the temp will go back down to normal. then youll need to fill up the coolant reservoir again. you need to be sure your getting heat from the climate control system this means the air has been bled out of the system...

now you could have another problem causing your overflow.
a head gasket could cause that too.
cooling hoses could be installed incorrectly to and from the reservoir. so when you got the water pump put in..the problem immediately occured when you left the repair shop?