If you have spring brakes you can actually have bad brake chambers that appear to be a leaking valve.Most brake valves aren't all that expensive if you buy remanufactured valves. The problem I have with remanufactured valves is they paint over the label and everything so you no longer know what the part number is if you need to replace the valve in the future. Some valves can have many different part numbers depending on crack pressure and such.
(snip) I used a (six wheel truck) schematic from Bendex to redo all my air brake system.