Wednesday, October 17, 2018 Detailed Auto Topics
You May Also Like
AGCO Auto Quick Tip:

Try our new Category View for Detailed topics segregated by their topic.

Now it's even easier to find the information you need.

AGCO Auto Quick Tip:

You can also receive our Detailed Auto Topics, delivered to your computer, as soon as they are written.

AGCO Automotive Detailed Topic Blog

Please click this image to subscribe.

Detailed Topics

A brake pedal that is lower than normal is annoying and dangerous. Finding the cause can be tricky, but is much easier with a logical approach.

In part one in this series, we explain how to test for a low brake pedal by blocking the brake lines. Please read part one before continuing with this article. The block-off method shows if the cause of a low brake pedal is at the wheels or the master cylinder. Brake block-off can also isolate which wheel is causing the problem. Once the area of the problem is isolated, finding the cause is far more simple.

Testing for brake master cylinder problems

Using the block off method, described in part one, we can test for a master cylinder problem. With all wheels blocked off, if the brake pedal is still low, the brake master cylinder is often the cause. This can also be misleading, as the hydraulic control unit of the ABS system can also cause the same symptom. We first closely inspect the master cylinder for external leakage. We should replace any brake master cylinder that has a brake fluid leak. This saves further testing that may not be necessary with a known-bad part. Even without external leakage, the master cylinder can still be bad.

Internal seal failure in brake master cylinder

Internal seal failure may not cause a loss of brake fluid to the outside. Pistons in the master cylinder move forwards when the brake pedal is pressed. This pushes pressurized fluid into the system. If the seals on the master cylinder pistons leak, fluid will flow passed and back to the reservoir. This will NOT cause a loss of brake fluid. The brake pedal will slowly sink as fluid bypasses the seal. If we pump the brake pedal, the height may temporarily come back up. Holding steady pressure on the pedal will cause it again to sink.

Block off plugs to test brake master cylinder

To test the master cylinder for internal problems we plug the two brake line-ports with threaded plugs of the proper style. Finding the exact size and tread pattern can be difficult. We need to use great care as many different thread styles and pitches and sealing methods are in use. Using the wrong plug can ruin the threads and the master cylinder. Specialized kits are available with an assortment of block-off plugs, for testing master cylinders.

If the pedal is still mushy or slowly sinks under pressure, with the outlet ports blocked, the master cylinder is bad. With the ports plugged, if the brake pedal is very high and firm, the problem is not with the cylinder.  A pedal that remains low, with all wheels blocked and the master cylinder confirmed-good, suggests a bad ABS modulator. The ABS brake modulator will normally require professional testing to confirm.

Damage to the master cylinder from improper bleeding

Failure of the internal seals sometimes occurs after the brakes are improperly bled. Normal travel of the pistons and seals in the master cylinder are very limited. In a poorly maintained system, this allows corrosion to form in the area of the cylinder that is not used. When the brake lines are opened, the pistons travel much deeper into the cylinder. Corrosion can cut the seals and cause failure.

The brake pedal should never be pushed to the floor when bleeding brakes or with the lines open. Placing a block of wood under the pedal limits the travel and can help prevent this damage. It is also wise to vacuum old fluid out of the reservoir and replace it with fresh fluid, before beginning any brake service. This helps keep debris out of the system and prevent damage to the master cylinder.

When replacing a brake master cylinder, we may need to adjust the push rod from the brake booster. This is easiest with the tool made for the purpose. Failure to properly adjust the push rod can result in a low pedal or brake lockup. Much more information and pictures are available in the Detailed Topic on Diagnosing Brake Lock Up.

Mechanical problems that cause a low brake pedal

With the block-off test method, a firm pedal after blocking off the wheels means the problem is not with the master cylinder. Next we would release the line block at each wheel, one at a time. After releasing each block-off, we test the brake pedal height. If the pedal drops significantly, after removing a wheel clamp, we know a problem exists with that wheel.

We should replace the line clamp and continue testing until all wheels are checked. Multiple problems sometimes exist and incomplete testing will lead to confusion.

Calipers are sometimes installed on oposite sides and cause a low brake pedal

An example of a mechanical problem is brake calipers installed on the wrong sides of the vehicle. Many brake calipers will bolt onto the left or right. Placing them on the wrong side causes the bleeder screw to point down. Air will not flow down and bleeding the brakes will not remove trapped air. This is an easy mistake for an inexperienced person to make. Moving the brake calipers to the proper sides corrects the low pedal.

When purchasing brake calipers or wheel cylinders, make note of the part numbers and write left and right on the boxes. This can help prevent confusion. Also remember that the bleeder screw will always point up when we properly install calipers or wheel cylinders.

Diagnosing drum brake problems

As brake shoes wear, the distance between them and the drums increases. This extra distance will be consumed by the brake pedal. Since there are four rear pistons, the problem is increased four-fold.

Under normal operation, most rear brakes are supposed to be self-adjusting. With duo-servo brakes, adjustment took place when applying the brakes in reverse. With the far more common leading-trailing brakes of today, this is not so. Adjustment takes place when the parking brake is used. Many people do not use their parking brake and rear shoes can get out of adjustment.

Brake shoe proper adjustment

There will normally be a treaded device used to adjust brake shoes. Proper adjustment is .015 of an inch, between the shoe and drum. Without a tool, this can be difficult to measure. On non-drive wheels, a fairly close adjustment can be gauged by spinning the wheel. When released, the wheel should rotate about one and a half turn and stop. Less rotation means the brake is too tight, more means too loose.

Worn or bent backing plates can cause poorly adjusted brakes to feel okay. This can result in a low brake pedal, even after adjustment. For much more information on checking backing plates, see our Detailed Topic on Diagnosing Drum Brake Problems.

Disk brake causes of a low pedal

If the problem is with a disk brake, the cause will often be excessive piston travel. Caliper pistons are several times larger than the pistons of the master cylinder. Hydraulic advantage will greatly increase pedal travel when caliper travel increases.

Affect of wheel bearing slack on brake pedal height

Brake rotors are held in alignment by the wheel bearings. Slack in the bearings will allow the rotor to move and push the caliper pistons into their bores. When the brakes are applied, the pistons have to travel much further, causing a low pedal. A modern bolt in hub bearing should have no discernable slack. Any movement in the bearing means it is failing and should be replaced. Older style adjustable bearings must be properly adjusted.

Seized caliper sliding causing low brake pedal 

Another common problem with calipers is seized slide pins. Sliding calipers are designed to move using lubricated slide pins. When a pin seizes, the bracket that supports the caliper will flex, greatly increasing movement. When the pads are removed, the caliper should slide very freely on the pins. Any binding or resistance to movement means the pins should be serviced or replaced.

Brake rotors with excessive run-out will also reduce brake pedal height.  When a rotor moves side to side, the caliper pistons will be pushed into their bores.  When applied, the pistons must travel further to contact the rotor.  This additional movement can cause the brake pedal to be low.

Air entering the brake system

Any air in the hydraulic system will compress and absorb pedal height. There are many methods and patterns for bleeding brakes, and some require factory scan tools to bleed. Before attempting brake repair, consult the service information for the vehicle, about bleeding procedures. This can help avoid having to tow the vehicle to a shop. Another good practice is to avoid allowing air into the system, by keeping the reservoir full with fresh fluid.

Pushing pistons in, without expelling the old fluid, can damage the brake system

When installing new pads, pistons must be pushed back into their bores in the calipers. Before attempting this, the bleeder screw must be opened to expel the old fluid. Failure to observe this step will cause debris that collects in the caliper, to be forced back into the system. Failure of the ABS hydraulic unit may result.

It is important to push the piston in very slowly. Quickly moving the pistons can cause air to be drawn in, around the piston seals. Tighten the bleed screw before the piston is released. A loose bleeder screw may also allow air to be drawn into the system.

Brakes that keeping getting air in them

Wheel cylinders can leak air into the system

Sometimes, even after bleeding, wheel cylinders will still have air in them. This can result from worn or hardened seals. Ironically, the cylinder may show no evidence of leakage. Pressure from brake application spreads the seal, so it does not leak. When the pedal is released, air can be drawn past the weakened seal. This will show up as a low brake pedal, even after extensive bleeding.

Brake calipers can have the same problem, though it is not as common. Just because a seal does not leak fluid, does not mean it will hold air out. This problem is far more acute when using the improper sequence for bleeding. Bleeding the wrong wheel first can cause air to be drawn into the system. Always consult service information for the vehicle for the proper bleeding sequence and method.

 

 

 





Post or Read Comments (0)

Please click the link above to leave your comments

 

Registered visitors are always invited to leave their comments and thoughts by using the form above. If you need to you can login here or register here.

You can also win a free AGCO coffee cup, by reporting any errors you find, with this form.