A few years ago i flew a 172 with a similar problem. During a lesson i was demonstrating the pitch up moment caused by lowering flap and was surprised to see that a further, more exciting effect was fuel coming out of the wing airvents and dripping onto the avionics.
To cut a long story short, after a very quick return to the field it was found that the new wingtank was not in the possition that it should have been.
As a result if you had full, or nearly full fuel, at certain points during the flight fuel would slop around and come out and all the fuel above a certain level would end up coming out of the tank onto you or the hot avionics! It would be a VERY good idea to get an engineer to check that the tank is in good condition and that it is sitting securley in the possition that it is intended.
Good luck and happy landings!