My air powered diaphragm pump is food grade plastic, not SS, but you should not have any issues if you do it right. After pumping the syrup, rinse it with hot water (back into the evaporator) and then run it until dry. If it runs dry for a minute you have removed enough of the water to avoid freezing damage.
If you will again be using it the next day or 2, you can leave the tail end of the syrup in the pump. The syrup will not freeze.
If the pump freezes in use, you need an air drier on the air line.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.