Reservoir Connection: 3/4" Pipe Thread With Anti-Compounding The SR-5 is a trailer spring brake control valve. It mounts to a reservoir on the trailer and like the SR-2 & 4, it controls the trailer spring brakes. The SR-5 is similar to the SR-2 and SR-4 but it allows for simple trailer system plumbing while complying with the latest version of FMVSS 121 (Docket 90-3, Notice 4). However, unlike the SR-2 and SR-4, the SR-5 controls trailer spring brake release using trailer Supply Line pressure Because dedicated or isolated reservoir pressure is no longer required by law for spring brake release, the SR-5 system needs only one reservoir for service volume on single axle trailers. Two reservoirs are generally needed for 121 service volume requirements on tandem units. An integral anti-compound feature is part of the SR-5. Older trailers using the SR-4 can easily upgrade to the SR-5 with minimum re-piping. Pro Trucking Products Aftermarket 109615 SR-5 Trailer Spring Brake Valve, Reservoir Connection 3/4" Pipe Thread with Anti Compounding