when you say top off, you are just filling so that the float rises to the top correct? if you fill it to the top of the expansion tank, the radiator cap will allow that extra coolant to overflow once it gets hot enough and you'll end up where you are supposed to be after a while. That's really hard on the cooling system though.
You might be able to rent a pressure tester from autozone for free. Then to test, fill the tank to the correct level, and pump it up to around 20psi and see if it holds. If it isn't holding pressure, look for the leak. The cooling system might need to be warm (hard to test while hot) in order for the leak to appear.

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Olympic Peninsula, WA, USA
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2005 Mini Cooper 100k, British Racing Green/Silver-Black
04/2002 4.6is 140k, Silver/Black-Alcantara *FOR SALE*
1989 325iX 270k, Diamond Black/Black
1984 318i 370k *SOLD* (First Bimmer)
Honorable Mentions: '71 2002, '87 325, '90 325ic, '92 525it, '93 325i, '94 530i