The Bendix+ SB-4 (often associated with the Di-Pro+ line in older terminology) is a high-performance double-diaphragm spring brake chamber designed specifically for cam-foundation drum brakes on heavy-duty commercial vehicles. The double diaphragm spring brake chamber combines service, parking, and emergency braking into one unit. Often referred to as a -piggyback- chamber, the two separate brake chambers are physically stacked on top of each other to share a single pushrod.