The expansion tank tends to crack through the cap sealing area, venting the pressure into the overflow tank and then onto the ground. The fact that you don't get coolant pulled back from the overflow tank makes me think either your new cap is bad or the seal seat is bad on the tank.

Expansion tank on my TE was replaced at about 150,000 miles.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!