I've had two epansion tanks from GSF for my 1990 16v. The first one was an exact replica of the original VAG one i was replacing.
Within three weeks it started leaking at the seam at the circumference of the tank, back it went and i got an after market one that was almost transparent. Not had a problem since. However the electrical connection for the level sensor is easy to miss align with the connection on the car side.
I recommend checking that the two contacts in the expansion tank are straight and level before connecting the plug.