I would suggest a 7/8 or 15/16". I used a 1&1/8(manual) & it was bad-poor leverage, it took too much leg/ not enough travel. I would add a disc proportioning valve(there are several stock variations) rather than adding an aftermarket adjustable prop valve in the rear line with your stock drum prop valve. I defer to Dr Diff for the final word on the best selections for you. But from the tone of your post it sounds like your linings/pads/friction surfaces ain't right that you dont have enough braking even when you stand on it.