Right now you haveAn on/off petcock. It is just that the Prime position is “on”, and the On position is “off”. The only risks you have are:
1. There is no reserve position because the RES and PRI positions draw from the same level. And
2. If the diaphragm fails, it will leak fuel out of the vacuum barb, but that would be a tell tail that the diaphragm had failed. You could put a vacuum cap on the vacuum barb, but you’d lose the tell tale. But the chances of a failure on a diaphragm that isn’t being used is small. It won’t be in contact with the fuel that deteriorates it.