How Does a Line Lock Work? Here’s What Actually Happens

You've probably seen it at the drag strip.

A rear-wheel-drive car is sitting almost completely still. The engine comes up, the rear tires start spinning, smoke starts rolling—and somehow the car isn't driving through the front brakes.

That's a line lock doing its job.

A line lock uses an electrically operated hydraulic solenoid to hold brake pressure in the front brake circuit while allowing the rear wheels to turn.

I've got a Hurst Roll/Control installed in my car, so instead of just explaining how it works, I'm going to show you exactly what I'm doing behind the wheel—and what the car is doing at the same time.

 

In the video, you'll see the entire process from several angles: outside the car, through the driver's window, inside the car and from my point of view behind the wheel.

You'll see me apply the brakes, activate the line lock, take my foot off the brake pedal and then spin the rear tires while the front brakes hold the car.

Once you've seen that happen, the rest of the system is pretty easy to understand.

What Is a Line Lock?

A line lock is an electrically controlled hydraulic valve installed in the vehicle's brake system.

The basic idea is pretty simple.

You apply the brakes normally with the brake pedal. While that hydraulic pressure is applied, you activate the line-lock solenoid.

The solenoid traps pressure in the controlled brake circuit.

On my rear-wheel-drive application, that means the front brakes remain applied even after I take my foot off the brake pedal.

With my foot no longer applying the regular brakes, the rear brakes are released and the rear wheels are free to turn.

The front brakes hold the car.

The rear tires can spin.

That's what allows the car to perform a stationary burnout.

How Do You Operate a Line Lock?

Here's the basic sequence in my application.

With the car stopped, I apply the brake pedal normally.

While holding brake pressure, I press and hold the line-lock button.

That energizes the solenoid and traps hydraulic pressure in the front brake circuit.

Now I can take my foot off the brake pedal.

The front brakes remain applied, but the rear brakes are released.

From there, I bring up the engine RPM and release the clutch.

The rear tires begin to spin while the front brakes hold the car in position.

When I'm ready to move, releasing the line-lock button releases the trapped front brake pressure.

In simple terms:

Apply the brakes → activate the line lock → release the brake pedal → front brakes stay applied → rear wheels are free to spin → release the line lock.

That's the entire process.

What Is the Line-Lock Solenoid Actually Doing?

This is the part that sometimes gets misunderstood.

The line lock does not apply the brakes.

Your foot does.

When you press the brake pedal, the master cylinder creates hydraulic pressure in the braking system.

The line-lock solenoid is essentially an electrically controlled valve.

Once you have brake pressure applied and activate the line lock, the valve closes and holds that existing hydraulic pressure in the front brake circuit.

That's why the order of operation matters.

You don't push the line-lock button and have the front brakes magically apply themselves.

You create the pressure first. The line lock holds it.

Release the button and the valve opens, allowing the trapped pressure to release and normal brake operation to resume.

Once you understand that, the whole system becomes pretty straightforward.

Roll Control vs. Line Lock: What's the Difference?

This is where the terminology can get confusing.

The system installed in my car is sold as a Hurst Roll/Control.

So why am I calling it a line lock?

Because the same basic hydraulic brake-holding system can be used for both functions.

In my Roll Control demonstration, I showed how trapped brake pressure can temporarily hold the vehicle.

Roll Control vs. Line Lock: What's the Difference?

With a line lock, we're using that same basic principle differently.

Instead of simply demonstrating how the system can hold brake pressure, we're specifically holding the front brakes while allowing the rear wheels to turn.

Same basic hardware.

Same hydraulic principle.

Different use.

Why Use a Line Lock for a Burnout?

For a rear-wheel-drive car, a burnout means the rear tires need to spin.

Here's the problem.

If you're holding the regular brake pedal while trying to do that, you're applying the rear brakes at the same time you're asking the engine to overpower them.

Those two things are fighting each other.

A properly configured line lock changes that.

The front brakes hold the car while the rear brakes are released.

Now the rear tires can spin without the engine having to fight rear brake pressure at the same time.

At the drag strip, that makes it possible to perform a controlled burnout to prepare the drive tires before a run.

There's another reason I prefer doing a burnout with the car essentially stationary rather than carrying wheelspin down the road.

That's a separate subject—and I'm going to demonstrate and explain that one next.

The Hurst Roll/Control Parts and Installation Details

The unit installed in my car is the Hurst Roll/Control, part number 1745000.

Despite the Roll/Control name, this is the same unit I'm using to perform the line-lock function shown in the video.

There's one thing worth knowing before buying one:

The Roll/Control unit and the installation kit are sold separately.

Hurst Roll/Control — Part #1745000

This is the main electrically operated hydraulic solenoid system I'm using in the car.

Hurst also sells a separate installation kit containing brake-line hardware and fittings for compatible SAE-thread applications.

Hurst Roll/Control Installation Kit — Part #5671510

Don't assume the installation kit contains everything required for every vehicle. Brake systems, fittings and configurations vary, so verify compatibility with your particular application and check the current manufacturer's instructions before ordering.

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Installing a Line Lock

A line lock is simple in principle.

Installation is another matter.

You're modifying the hydraulic brake system, and that's not a place for shortcuts.

Brake-line routing, flare types, fittings, mounting, electrical connections and bleeding all need to be correct.

The system should be thoroughly checked for leaks and proper brake operation before the vehicle is driven.

You also can't assume that because an installation works on one car, it will work the same way on another.

Vehicles can use different hydraulic layouts, ABS and other brake-system configurations that affect whether a particular line-lock system is appropriate and how it needs to be installed.

Follow the manufacturer's instructions for your exact application.

If you don't have experience working with hydraulic brake systems, this is one of those jobs I'd leave to someone who does.

A Line Lock Is Not a Parking Brake

This is also worth making very clear.

A line lock is a momentary brake-holding device. It is not a parking brake.

Never rely on one to secure an unattended vehicle.

It's a performance component designed for a specific job, and it should be treated that way. Burnouts should be limited to appropriate controlled environments, with the risks to people, equipment and the vehicle taken seriously.

Why I Wanted to Show You Instead of Just Tell You

I've been working on cars for more than 40 years, and I've always thought mechanical systems make more sense once you can actually see them work.

A line lock is a perfect example.

I can tell you that a solenoid traps hydraulic pressure in the front brake circuit.

That's technically correct.

But watch my foot come off the brake. Watch the front of the car stay planted. Watch the rear tires start turning.

Now it makes sense.

That's why I used multiple cameras for this demonstration.

You can see what I'm doing with the controls and, at the same time, see exactly how the car responds.

Sometimes seeing it happen is the best explanation there is.

What's Next?

We've now covered both sides of the system:

Roll control and line lock.

But there's one more part of this conversation I want to get into.

You've probably seen guys do rolling burnouts with the tires spinning while the car is already carrying speed.

I prefer keeping the car essentially stationary and controlled.

Why?

That's the next video.

And there's more to it than making a better smoke show.

—Troy Kane
V-Twins to V-8s
40+ Years of Hands-On Automotive Experience

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