my 04yfz rear brakes dont work, i bought the quad used, i know it sat for a while. not sure how to begin troubleshooting to find what the prob is.

Start by checking the condition of your brake pads. then bleed the brakes.

check to make sure the rear rotor isn't bent a little.

try bleeding them, other problems could be the pads, rear master cylinder. an issue i recently had was the the pins the brake caliper slides on were seized. it was a pain in the ass getting them out but once i did and cleaned them up they worked fine. if there was some moisture in there and it wasnt used for a long time i could see that being your problem. but anyway start by bleeding the brakes

banjo fittings, line, caliper, pads, rotor, or master cylinder