I have a 2004 Marine master tandem axle trailer and have never replaced the brake pads. They don't appear to be worn out and still have pad thickness, however the last couple of years it doesn't seem as thought the trailer braking is working well. How often should a person change the brake pads on these? Is it like a car where there is no need to replace until they are almost totally worn out, or should it be done after so many years regardless of remaining pad thickness?