Determining Cause of Low Brake Pedal - Line Lock Test

Determining Cause of Low Brake Pedal - Line Lock Test

When it comes to low brake pedals, there are many possible causes. As an example, the list below shows the most common causes for low brake pedal:
  • Air in the hydraulic system
  • Rear brake adjustment
  • Fluid leak (pressure)
  • Internal bypass (i.e., RWAL/RABS dump valve bypass)
  • Seized slider mechanism
  • Excessive clearance between brake pads and the rotor
  • Binding or cocked brake pads
  • Self-adjustment in the calipers is not taking place as designed
Although this list is not all-inclusive, it does represents the most common causes. Likewise, it demonstrates why you should use a systematic approach when diagnosing a low brake pedal complaint.

Low Brake Pedal - Solution

2 Line locks

Systematically approaching the problem will enable you to pinpoint what part of the system is responsible for the extra pedal travel. As you use this method, you will begin to focus your attention on one area rather than taking a random or shotgun approach to the problem. The most effective way to start the process is with what is known as a line lock test. A line lock (pictured above) is a device that allows you to pinch a brake hose off without damaging it. Installing line locks will let you break a rather complex system down into smaller parts.

On rear-wheel-drive vehicles equipped with a front to rear split hydraulic system, installing 3 line locks will break the plan down into four parts, each front wheel, the rear axle circuit, and from the line locks up to the master cylinder see below. Four line locks installed on a front-wheel-drive diagonal split system will break the brake system into five parts (Second image below). Each wheel and the rest of the system. To perform the line lock test, follow the steps below:

Front wheel drive schematic

Line-Lock Steps to Fix Low Brake Pedal

1. Inspection

Perform a visual inspection for pressure leaks that would cause the pedal problem. If you can't find any apparent cause, proceed to the next step. On the other hand, if you find a problem, correct it before moving on.

2. Install Line Lock

Install a line lock on each brake hose in the system (on multiple hose arrangements, only one line lock is necessary). Position each line lock about midway down the hose.

3. Test Line Lock

With the wheels off the ground, vehicle in neutral, have someone start the car and apply the brake pedal. Attempt to turn each wheel. If you installed the line locks correctly, the wheels should spin. If one or more wheels do NOT spin, the line lock has not held pressure. Go to step. If all wheels spin, go to step 5.

4. Adjust Line Locks (if necessary)

Release the brake pedal. Remove the line lock(s) on the wheel(s) ) that do not spin. Reinstall the line lock(s), making sure the locks are tight. Repeat step 3 until all wheels spin.

5. Testing the Pedal

Making sure the vehicle is running, apply and hold the brake pedal. If the brake pedal is rock hard after taking the free play out of the linkage, go to step 7. If the pedal is spongy or has excessive travel before it gets hard, go to step 6.

Note: With the line locks installed, the pedal should no longer be able to move. The regular movement of the brake pedal causes the movement of the caliper & wheel cylinder pistons. After installing line locks, there is nowhere for the fluid to go, so the brake pedal should be rock hard.

6.Spongy Brakes

If you've installed line locks and the brakes feel spongy, there is a problem in front of the line locks. The most common cause of this is air in one or more of the components. For vehicles equipped with the Kelsey Hayes rear-wheel or four-wheel ABS systems, the reason for the pedal travel could be a bypassing dump valve.

7. Below the Lock Lines

If the pedal is hard after installing the line locks, the hydraulic system is in good condition and functioning correctly from the line locks up. Excessive pedal travel comes from problems at one or more of the wheels. With the vehicle running and the brake applied, have someone remove the rear line lock if the car is a rear-wheel-drive or the right rear line lock if the vehicle is a front-wheel-drive.

Next, note the amount of pedal drop. When adding a drum brake axle back into the system, the expected pedal drop should be ¼” to ½” if adequately adjusted and no air is in the system. When adding one drum brake back into the system, the drop should be 1/8” to ¼.” If the pedal drop is more than expected, go to step 8. If the pedal drop is correct, go to step 9.

8. Pedal Travel

The most common cause of excessive pedal travel on drum brake systems comes from a lack of self-adjustment. The only other reason could be air from the line-lock down to the wheel cylinder. Either fix the cause now or reinstall the line lock and continue with the next step. When reinstalling the line lock, make sure the wheel has is isolated before continuing.

9. Check Pedal Drop (FWD vehicle only)

On RWD vehicles, proceed to the next step. On FWD vehicles, remove the left rear line lock and note pedal drop. If excessive, go to step 8.

10. Check Pedal Drop

With the vehicle running and brake applied, remove the right front line lock and note the amount of pedal drop. A typical disc brake should drop ¼” to ½.” Go to the next step if the pedal drops more than this amount. If the pedal drop is ordinary, go to step 12.

11. Pedal Drop After Adding Disc Brake

If you have excessive pedal drop when adding a disc brake back in the system. Check for the following:
  • Seized slider mechanism
  • Excessive clearance between brake pads and rotor
  • binding or cocked brake pads
  • Air at one or both front calipers
  • Self-adjustment in the calipers is not taking placed as designed
  • The vehicle has a QTU master cylinder and QTU valve is bypassing

12. Standard Pedal Drop

If you have a standard pedal drop after adding the right front wheel back into the system, repeat step 10 with the left front wheel.

The line lock is one of the most effective diagnostic tools a technician can have in their boxes and no technician should be without them.

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