I have a short in one of the wires for my early Simple Metal with a rev D board. The voltage makes it hard to replace and get consistent results. This is an easy z end stop solution using simple hardware that can act as a permanent replacement OR a quick fix while new parts are ordered.

A quick side note - you will need to be able to level your print bed, this can be done by shimming the bed you already have or by rigging up a bed that can be adjusted like many other printers do with springs - its up to you.

Solder the leads exactly as the rest of the other end stops on the printer and mount with 2.5mm machine screws.

You will want to remove the G29 probe command from your start up code since you won't need the software to level your bed anymore.  You do not need to change the firmware to use this with the Rev D board. If you have a Rev F4 board you will need to follow this guide: http://help.printrbot.com/Guide/How+to+use+switch+type+Z+endstop+on+Rev+F4/156

I made two versions. Depending on the material you use and how well your 3D printer is set up will depend on which one fits.  The 2.5mm machine screws will thread in perfectly to hold the end stop.