Many buses with hydraulic brakes will have a hydraulic power assist, often powered by the same hydraulic assist system for the power steering. The first thing to do is check the fluid level. It's rare these systems fail, but it can happen.
Assuming you find it low, add the appropriate fluid (usually any commonly available power steering fluid, but check the cap or dipstick or sticker on the pump to be sure). Start the engine, turn the wheel a couple times (all the way to either stop) to bleed the air out. Push the brake a couple times to do the same. Check again and top off as needed.
Now, assuming you find it full (or the above measure did not make any improvement), the most likely cause would be a failed pump or high-pressure line. The line will be obvious. It will be leaking fluid everywhere. The pump, not so obvious (some pumps "whine" with age, or low fluid) but hard steering, *AND* hard (hydraulic) braking? I'd be checking the pump.