I wanted to post this here for those out there with a 2004 or 2005 650 V2. The rear aux brake (foot brake) from the factory is poorly designed and fails on all of these models over time. You can't purchase the replacement master cylinder kit anywhere but from ebay and I don't think I would put that factory junk back on anyways. The fix is to install the 2006 configuration of the rear aux brake, so the separate master cylinder kit and brake caliper with pads which are both available on Amazon. I mounted the master cylinder kit behind the gas tank and zip-tied the line up. After messing with several configurations this one actually works I've been riding for the last couple months on it with no issues. Literally a 20 minute job, just make sure to bleed the brakes real good to get all the air out and adjust the plunger for the petal. Here are the links to both parts, hope this helps a couple folks out.

 

Nice info.

 

My brother-in-law has a 2005 650V2 and replaced with these parts this weekend and his works now as well!