Maybe the pad return springs are collapsing the wheel cylinders all the at down and the reason you need to pump so much is because that’s how much fluid is needed to push the pads all the way out to the drums
The system needs a residual line pressure valve that holds a the pads out with a fixed pressure but also the pads themselves should be adjusted all the way to the drums so that the return spring is almost home when there is no brake input
The residual line pressure valve is meant to match the return spring pressure
-s