Brake warning light after brake sys flush

[Austin C10]

Immediately after purchasing my 1972 LWB Cheyenne, I had it in Firestone getting 2 matching rear Firehawks. I also had them drain and "flush" the brake system. About a week later, I had a pretty hard stop coming off the highway into a traffic jam. Since then, my brake warning light on my dashboard won't go off. Additionally, the brake pedal needs to be pressed harder to keep the truck stopped at idle.

Indications point to unequal pressure between the fronts and rears. Other than returning to Firestone, what are the first steps that I need to take to re-equalize? What else could be wrong?

Thanks!