Water in tank

[Sergio SADF]

About 3 or 4 years ago the 1979 C337H I fly was sleeping outside the hangar and with a rain of about 40 mm (near 2 inches) I drained about 8 drain cups of water from teh left wing.
I notice the new to the mechanic and he said that fixed the problem, but last anual 2 weeks ago I got about 4 drain cup from the outside drain point of the left wing.
Does anybody know if there is any modification to avoid this problem again ?
Thank you