I have the misfortune of owning a 1998 GMC 1 ton dually, the rear brake drums have seals loacated on the inboard side that seal against a surface of the axle. As it is a full float axle, meaning there are tapered bearings in the brake drums the bearings are lubricated with axle fluid and the seals are there to prevent leakage of the fluid. The failure of a seal provides a fluid leakage path that is down the inside surface of the drum, across the surface of the drum which the brake linings contact and out between the backing plate and drum. I have had 3 seal failures on one side and 2 on the other. A failure of the seal allows 80W90 axle fluid to soak the break linings, as this is not obvious, the failure is noticed by increased stopping distance as the rear brakes are rendered useless by the contamination with axle fluid. Since 1-ton trucks are uasually sold to pull or haul this is an especially dangerous condition, with a full rated load the rear brakes are much more critical than that of a pass car. I talked to mechanics at GM dealerships and they say the seals have been a problem since as long as they can rember. I've since looked at the backside of the rear tires of dually and noticed the tell tale sign of a failed seal of axle fluid stains. Theres an awfull lot of 1 ton duallys on the road with no rear brakes.

- Gregory, MI, USA