line lock

A line lock is a hydraulic control valve installed in the brake system that allows a driver to lock only the front brakes while the rear brakes remain unpressurized. It is operated by a dashboard-mounted switch and serves a specific purpose: holding a vehicle stationary on an incline or during a rolling start, or deliberately locking the front wheels to generate heat and smoke in the rear tires for weight transfer and launch in drag racing.

The device works by isolating the front brake circuit from the proportioning valve. When activated, full hydraulic pressure from the master cylinder is directed to the front brake calipers while a solenoid valve blocks pressure to the rear brakes. This creates a locked-wheel condition at the front axle. The driver must release the switch before driving normally, as sustained front-only braking under acceleration will cause the rear end to slide unpredictably.

Line locks are primarily used in drag racing and street racing contexts. A competitor activates the line lock at the starting line, spinning the rear wheels against the locked front brakes to heat the tire compound and increase grip coefficient. The technique, called a burnout, can improve launch traction by 10 to 15 percent depending on track temperature and tire construction. Once the burn is complete, the driver releases the switch and both brake systems function normally for the race.

Installation requires plumbing modifications to the OEM brake system, including a solenoid valve, brake lines, and electrical wiring to the switch. Most aftermarket kits are designed for vehicles with disc brakes on both axles; systems with drum brakes on the rear are difficult to integrate. Pressure relief valves and check valves are often included to prevent system damage or unintended rear brake activation.

Line locks are illegal for street use in most jurisdictions because they compromise brake system redundancy and create hazards during normal driving. They are restricted to closed-course and sanctioned racing events. Some drag strips require a functional line lock as part of safety specifications for competition vehicles, while others prohibit them entirely. Always verify local regulations and track rules before installation.