The wheel cylinder seals are prone to leakage from deterioration. This condition may be checked on the truck without disassembly. After the truck is elevated and supported safely, remove the rear wheels. Remove the brake drum on each side.
Carefully lift the edge of each boot away from the body of the cylinder. Inspect the inside of the boot and the edge/end of the cylinder. You can expect a very slight (normal) moistness in the area — usually covered in dust — but any sign of wet fluid is cause for immediate repair. A leak, no matter how slight, can significantly reduce the braking effort on that wheel. It can also admit air into the brake system, causing poor pedal feel.