As you have installed the back flow preventer, I'm assuming that this installation was not inspected as a lead-free device is required for potable water service. But here are a couple of questions for you to think about.
Is the valve to control the water powered sump pump before or after the back flow preventer? If it is before the back flow preventer, then the only time the back flow preventer is "seeing" your water supply is when that valve is open, and the sump pump is operating. It's pretty hard for any lead ions to migrate upstream in a flowing water line. If it is after the back flow preventer, you could theoretically install a second identical valve before the back flow preventer.
Do you have copper pipe with soldered joints? How old is the piping? Do you know if the joints were made with lead-free solder?
I'm all for reducing health hazards, but in some cases, like this one for example, using lead-free devices seem to be a bit of overkill.
If you decide to leave the back flow preventer you have in service, you now know that is should have been a lead-free device, so if you sell your house, you will need to indicate that in your disclosure statement or remove/replace it at that time.
The above are not any specific suggestions from me, just something to think about. The original focus on reducing lead in drinking water was to eliminate lead supply piping. My brother-in-law in south St. Louis still had lead water supply lines throughout his house, as do thousands of houses in St. Louis and other older cities. Those houses are ones that DO need remediation IMHO. But worrying about lead-free brass in a back flow preventer at a dead-end water line does seem to be a bit much, again IMHO.
