Background: I have a 2007 Sportsman 800 EFI. It had the classic issue of the foot brake not working. There was enough corrosion on the rear caliper I decided to replace the rear caliper, foot piston, line, and reservoir. I fully bled all the brakes, it was time.

Issue: When using the using the brake handle there is no resistance, you pump it a few times and then the fluid leaks out the foot brake reservoir. This appears to either be:
1) Defective rear caliper issue, there is a leak between the pistons causing the fluid to mix and the hand brake to move pressure into the foot brake
2) Defective foot brake piston, the seal on the piston is actually supposed to keep back pressure from the main brake from pushing into the reservoir.

I would appreciate some expertise, is the rear caliper piston supposed to allow mixing of both the main and foot brake or is there a broken seal in the new part I bought?

 

That system uses 1 piston, with 2 seals on it that separate the front from the rear(hand and foot) brake. One of the seals is torn or cut is my guess. If OEM caliper get seal kit 2201872 and tear it apart. If aftermarket caliper, not sure if they are rebuildable. Its not just a typical o-ring in the caliper piston. They are "Square cut o-rings" so my guess is auto parts store will not have anything close enough to fix.