Cessna 172 fuel tank breather

I pressume that your problem occurs more during initial climb with full tanks.Since the model is old,my initial suspicions would be:

• Defficient closing of the non-return air valve which is located at the end of vent tube,inside the upper part of the opposite tank.
• Blocked filler cap vents which would permit excessive tank pressure to spill the fuel out .
• Cessnas air vent tube is notorious for loosing up to 5 Gals on the ramp.I found my health by substituting a modified version by McFarlane.

Whats your problem ?

What model is your C172?