Hy Dawso, so just to clarify you're saying your brake drums are 15 1/2 inches in diameter? Are you measuring the inside diameter of the drum, where the brake shoes ride? I cannot recall any brake drums that are 15 1/2 inches inside diameter. The single rear wheel cylinder is unusual, most rear wheel brakes of that era had two wheel cylinders per wheel for the larger trucks. Pictures of what you have would be very helpful. Metric dimensions on parts of that era did not exist here in north America. The wheel cylinder you have is a lot smaller in bore than what we are used to seeing, and as I said most systems of that size of truck use two wheel cylinders per wheel and much larger in diameter.
