This is the Firestone Coil-Rite Rear Air Helper Spring Kit, part number 4135. It installs inside existing rear coil springs and offers adjustable supplemental air springs for select Lexus and Toyota SUVs, including the 2013-2018 LX570, 1998-2021 4Runner, and 2007-2014 FJ Cruiser. It helps maintain ride height, control sag, and improve handling when carrying heavy loads.
Constructed from durable elastic polyurethane, this kit includes two air springs, all necessary hardware, air lines, and separate manual inflation valves. It supports a load capacity of approximately 500 to 1,000 lb per set, and operates within a pressure range of 5 to 35 PSI. Designed to function in extreme ambient temperatures—from -40 °F up to 180 °F—without requiring any drilling during installation.
In real-world use, this system helps you stay level front to back, reduce body roll, prevent bottoming out, improve braking effectiveness, and preserve steering response. It’s ideal for SUV owners who tow, carry gear, or haul passengers frequently and want improved stability and comfort without replacing their factory springs.