I would remove the sender and install a direct pressure gauge on the sender port. It will rule out wiring/sender/gauge. (Available at Home Depot etc,,don't fret if its
Oil proof or not and designed for water. Consider it expendable)
The new sender you have may be a "common ground" type and wrong PN for your gauge if it is the "Floating Ground" (variable ground) type. Thus the peg to 100psi you see now.
Since engines usually have a 24 mo warranty, I'd contact Backdraft if its a pump issue.