Psmith, can I ask why you want to make the change? Are you upgrading the rest of the drive line? If leaving the truck stock the mechanical brakes should work fine. They are the S-Cam type just like the Air Brake big rigs use today, except they are mechanically activated.
One advantage is mechanical brakes require less maintenance over time...let the truck sit for a year or two and as long as the shoes don't rust to the drums they will still work...can't say that about hydraulic brakes.
Mike B
