Your unit with a Spartan chassis has automatic slack adjusters.
They need to be calibrated periodically which is done from the driver's seat.
This involves fully stroking your brake pedal for 30 seconds or so, repeat 5 times.
Roll down your window, if you hear a clicking sound then your slacks are adjusting to restore your full braking power.
Often overlooked, simple to do and does not cost you a dime!
Videos are available on the web.
I would try this first.