While you are at it make certain it is plumbed correctly. The rear end of the master (large section) goes to front brakes. Likewise, the prop valve is front/ rear sensitive.
Yes, this ^^^
Plus, keep in mind that contrary to popular belief, the front brakes do more than 70% of the braking. Too much rear bias, and YOU. WILL. SPIN.
As far as the brake light coming on, it sounds like you're either low on fluid or there's still air in the sysem. Most likely the latter.