I'm not sure which one Junky is referring to as I don't know of any with low fluid sensors since those are typically in the master cylinder reservoir, however there are several distribution blocks on the market with built-in adjustable proportioning valves and a pressure sensor port. The pressure switches used on these setups do just that, sense pressure, and could be used to activate tail lights in an application without an electro-mechanical pedal switch, although there was a pretty decent thread recently that makes a pretty strong argument for the reliability of the pedal switches over the pressure switch.
Wilwood #
260-11179
Summit #
SUM-G3910
SSBC #
A0730
Tobin
KORE3