First, welcome to the best place on the web.

I presume what you are talking about is the pressure activated switch on some of the newer adjustable proportioning valves.  You'll need to extend both wires from your stock brake light switch through the firewall to the pressure switch. 

Sorry, I just googled MBM and I see that what you are probably talking about is the differential switch that is common on all late model prop valves.  That is a switch that applies a ground to a brake warning light on the dash.  Normally, if you want that light, you'll need to install a lamp in the dash or somewhere where you'll see it.  I used an LED with one side hot from the Ign. switch and the other side connected to that switch (either terminal).  If you spring a leak on the brake system...front or rear...that switch will be closed by differential pressure in the prop valve and will turn on the light by applying ground.

If you choose to ignore the switch it will have no effect on brake performance.

BB
 

Last edited by Bullet Bob (2/22/2019 10:38 AM)

The adjustable valve (with knob on it) has a pressure switch to connect to the stoplight switch.  As Bullet originally stated, just remove the two wires from the original pedal mounted stoplight switch and run them to the two wires on the pressure switch.  It does not matter which wire connects to which switch wire.  It is simply a normally open inline switch.  Any sensed brake pressure completes the circuit.