As for how the primer works the white fitting in the above linked item is the return line and black fitting is the pickup line. As press the primer bulb air/fuel is discharge through the return and as you release fuel is pull into carburetor via the pickup line.
In the last few years I have been having more than a fair share of these flanges (primer bases) to be failing and they cost nearly as much as complete replacement carburetor. Personally Stihl really has mess up Zama by barring them selling anything that is Stihl used only items. This has really been a boom for after market suppliers like DC Spares and Carb Tech but they have yet to cover everything,.
