It would seem that on systems that have a return fuel line, and the system pressure is regulated on the return flow, that the return would have to be blocked, so that the rather small volume of fuel available in the canister is not sent to the vehicle tank.
In that case, fuel pressure control would be accomplished solely by pressurization of the canister.
In the case of K-Jetronic systems the required fuel pressure will vary from ~75 psi to ~95 psi, depending on system version.