Modern Vespa

As the title states, I'm looking to add an aux tank to my GTS for Cannonball. It needs to be easily installed and easily reversed. I've scoured older threads here on MV and elsewhere on the web, and hoping I can get definitive guidance from the experienced folks here.

I'm planning a simple gravity feed solution, with the tank on my rear rack. I plan to remove my top case for the SCR. I've found a 2.25 gallon (there is also a larger 3.5 gal) aux tank with vented cap and outlet for a vent tube as well.

I would like to be able to control the flow of gas from the Aux tank with an inline valve (like a reserve switch) and plumb to a 90 degree elbow fitted to a drilled gas cap on the main tank. Fuel would flow from the Aux tank to the Main tank through the modified cap. An inline quick-disconnect would allow me to disconnect the end with the modified cap to allow for refueling of the Main tank.
The vent tube from the Aux tank would connect to the Evap canister via T-connector in the existing evap line.

My primary concern: is this too simple a design? As the fuel level in the Main tank drops, will this impede the flow of fuel from the Aux tank?

Any suggestions would be appreciated.

Think you may have to leave the Aux vent open to environment.
Give it a try.

I am far from any expert but just a thought on pressures. The aux tank is metal, will be mounted out in the sun and getting warm. This would cause some expansion and pressure increase in tank, although small but since the fuel tank is plastic and not exposed, could the aux tank back pressure the main tank with a negative effect on fuel delivery?

Not sure if Bagel aka Tom Donahue is on this forum, but he has ridden to hell and gone more than once on his GTS, and he has an aux tank in the top case. It would be worthwhile to pick his brain.

Looks relatively complex to me. Fuel cannot transfer from the auxiliary tank unless it is vented so I would only have one valve for venting the aux. tank.
Ride scooter until fuel gauge shows xx% level or the fuel light blinks then simply open the vent and the fuel will flow into the main tank and then show on the fuel gauge. The rate of flow between the aux . and main tank is irrelevant as long as the fuel line is normal size.
If you have a aux. tank larger than the main then shut off the fuel when the main tank shows yy% or full.

Did this in 2012, 5 gallons, kept the weight low with a custom mount, worked great.
The fuel was plumed *THRU* the left rear fender(The valve at the end of the yellow fuel line.) as shown in the first photo to the underside of the main fuel tank and both vent lines tied together.
7.4 gallons will take you a long long way.
Could be for sale...

The way to do it. And it does not have to be completely full.