[Gaz64]

Adding a 50 exhaust discharge from a 00, is like adding a third pump nozzle, and so 33% will go back to the bowl at a heavy stab. At very slow applications, generally a lot less gets to the nozzles.
I wouldn't jump from 00 to 50 in one hit. I would start at 35.
If you buy steel ball valves, you can either drill the 35s to 40,45,50 etc, or solder them closed if you make a tuning mistake.
Once you find your happy spot, you can replace them with your new ones of your tuned size.