2013 Assault - No Brakes...is there rebuild kit?

These system are super simple. I doubt either of them need rebuilt. Start by gravity bleeding it if you cant get any pressure to build up. Remove lid from master cylinder, and remove the bleeder screw completely. Add brake fluid and just let it set for a few minutes to see if fluid will flow on its own.

Once you get fluid flowing, dont let the master run out of fluid and suck in air. Reinstall bleeder screw. Then bleed the system. Bleeder closed, slowly pump the brake lever 2-3 times and make sure you let it release all the way, then hold it to the bar (and keep holding it) and loosen the bleeder screw. Fluid should flow out the bleeder for a second or 2, when it stops tighten the bleeder screw. Now you can release the brake lever, add fluid if needed, and repeat.

Thoroughly wash off any brake fluid with warm water when your done.

I've had two of these master cylinders fail internally, they just go flat to the bar, Polaris replaced both of mine under warranty. If you have an IQR master cylinder, that is the same masters cylinder as the PRO uses I would replace it and go from there.