This is the part of their conversation to 1/4 turn the upper housing counter clockwise to access the piston.
Just an update for all who have leaking porcher veneto toilets.. I have a 11-year old porcher, so I called Porcher at 1-800-359-3261 in regards to my leaking veneto.. They transferred me to a technical support specialist who said I either needed a new flush piston, aptly referred to as a piston assembly 83084.00.000 by ha1976, or a new membrane. They sent me both..
My leak was due to the piston assembly (flush piston). The piston sits inside the upper valve housing. The upper valve housing comes off by twisting the upper housing counter-clockwise. When you lift the valve housing out of the tank you will see the flush piston extruding from the bottom of the housing. The flush piston can be pulled out of the housing.. Yes, it just pulls out. And you just push the new piston in place.. Secure the valve assembly back into the tank by rotating the housing clock-wise.. By the way, my veneto has two clips on the housing which secure the housing in place.
Hope this helps. And thanks to everyone on this site who gave me enough info to fix my leaking veneto.. Cheers to the tech specialist at Porcher who helped me too.