98 Blazer brakes

Hi my aunts 98' Chevy blazer is giving us a hard time. the ABS light is on and there is a lot of travel inthe brake pedal with little effect. there is also a hissing sound that gets louder the harder you brake. I have to floor the pedal to get the brakes to engage. we have bleed the brakes but i have read that you need a special tool to do it properly. Any and all advice would be appreciated

An ABS capable scan tool is necessary to retrieve the DTC's and bleed the brakes.