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Brake Assist: Diagnosing Low And Hard Brake Pedals

Brake pedal assist complaints are often subjective in nature and, for that reason, can be difficult to solve. Grandma, with her arthritic feet, might be highly sensitive to a slight loss of pedal assist, but her lead-footed grandson not so much. But on-going technology is bringing a certain degree of objectivity into brake-assist diagnosis.

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Brake pedal assist complaints are often subjective in nature and, for that reason, can be difficult to solve. Grandma, with her arthritic feet, might be highly sensitive to a slight loss of pedal assist, but her lead-footed grandson not so much. But on-going technology is bringing a certain degree of objectivity into brake-assist diagnosis.

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To illustrate, I was recently involved in a mobile diagnostic case involving a 2000 Toyota Land Cruiser that, unlike most vehicle applications, uses its anti-lock braking pump and accumulator to provide brake pedal assist for normal braking. The Land Cruiser’s brake assist (BA) system can also interface with an anti-lock braking system (ABS), vehicle stability control (VSC) and traction control (TRAC) system, if so equipped.

While these systems and their networking issues can complicate the diagnostic process far beyond the scope of this article, we nevertheless can illustrate how this brake assist system can be diagnosed by reading trouble codes and following appropriate diagnostic procedures, instead of relying on subjective opinion.

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The Land Cruiser’s electronic brake control module had stored diagnostic trouble codes C1210 (yaw sensor calibration undone), C1223 (ABS system malfunction), C1252 (hydraulic brake booster motor malfunction), C1256 (accumulator low-pressure malfunction) and C1336 (zero point calibration of deceleration sensor undone). The initial symptoms included intermittent loss of brake assist and then progressed to complete loss of brake assist. Initial research indicated that the pump motor is a frequent failure point.

Upon inspection, the ABS pump was cycling on and off with the ignition in the run position. As the system warmed up, the ABS pump began to run continuously, indicating that the pump couldn’t build enough accumulator pressure to turn off the pump motor. One diagnostic scenario was internal leakage in the accumulator, while another was leakage through the various ABS actuator valves.

Photo 1: The ABS accumulator assembly and fluid reservoir are the only visible parts of Toyota Land Cruiser’s hydraulic brake assist. The ABS module is hidden at the photo’s lower right corner.

Photo 1: The ABS accumulator assembly and fluid reservoir are the only visible parts of Toyota Land Cruiser’s hydraulic brake assist. The ABS module is hidden at the photo’s lower right corner.

Keep in mind the Land Cruiser’s hydraulic brake assist is serviced only as a single major assembly or two sub-assemblies consisting of the motor/accumulator or actuator/master cylinder assemblies, so it’s not always necessary to diagnose individual components. The cost of the complete assembly is in the $2,400+ range, so it’s not an inexpensive repair. See Photo 1.

CLEARING THE CODES

With the initial codes stored in my scan tool, I decided to clear the codes to determine if we had a hard or an intermittent failure. Key-on, engine off, codes C1223 (a general code indicating the presence of an ABS problem) and C1256 (indicating a loss of accumulator pressure) reappeared. I ignored the C1210 (yaw sensor calibration undone) and C1336 (zero point calibration of deceleration sensor undone) for the moment since a diagnostic/reset procedure is included in Toyota’s service information (SI). See Photo 2.

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As for code C1256 (low accumulator pressure), we found several inaccuracies in aftermarket information. The OEM Toyota SI for C1256 includes six pages of testing procedures which can’t be summarized in this space. In brief, Toyota SI outlines a test method using a brake system pressure gauge on each brake caliper.

Photo 2: Code C1256 indicates that the accumulator pressure is low and that the failure is a hard, rather than an intermittent fault.

Photo 2: Code C1256 indicates that the accumulator pressure is low and that the failure is a hard, rather than an intermittent fault.

On our Land Cruiser, the motor/accumulator assembly was evidently failing to recover accumulator pressure, which set the C1256 code. We took a diagnostic short-cut by testing pressure directly at the accumulator outlet. As indicated by code C1256, the pump was producing no significant accumulator pressure.

Additional testing indicated that the B/A, ABS, VSC, and TRAC electronics appeared to be fully functional. At this point, I recommended replacing the hydraulic brake assist unit as a complete assembly to avoid any warranty/liability issues. After extensive bleeding, the result was a successful repair. See Photo 3.

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TESTING VACUUM BOOSTERS

Conventional brake assist or power brake boosters typically used in Toyota and other nameplate passenger and light truck applications are operated by intake manifold vacuum. When dealing with hard-pedal braking complaints, vacuum-assisted brakes should be tested by first removing the check valve from the power booster with the engine off. Lubricating the check valve grommet with aerosol silicone will ease removal of the check valve.

Photo 3: Circuit testing can be done at the three connectors mounted on the ABS module at the driver’s side of the hydraulic brake assist assembly.

Photo 3: Circuit testing can be done at the three connectors mounted on the ABS module at the driver’s side of the hydraulic brake assist assembly.

The booster should be holding vacuum. If not, check for vacuum leaks at the front and rear pushrod seals and check the valve grommet with the engine running. I use a length of heater hose as a stethoscope to check for hissing noises at either pushrod or the check valve grommet with the engine running and as the brakes are applied and released. If a vacuum leak is detected, repair as needed. Next, inspect the interior of the vacuum booster for brake fluid. If the vacuum chamber is full of brake fluid, the rear master cylinder seal is leaking and both the booster and master cylinder should be replaced.

Next, start the engine and test for air flowing freely through the check valve and hose. If air flow doesn’t affect idle performance, inspect the intake manifold hose nipple for carbon accumulation and inspect for a clogged hose or faulty check valve. With the engine running, test intake manifold vacuum at the booster supply hose. Intake vacuum should be within the 18”Hg to 22”Hg range at sea level. At 8,000 feet altitude, intake vacuum should range between 12”Hg and 15” Hg.

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If intake manifold vacuum is low, the engine might have retarded valve timing, a major vacuum leak or clogged exhaust. If the vacuum booster passes all of the above tests, discharge the booster by pumping the brake pedal until all vacuum assist is discharged.

Last, step on the brake pedal and start the engine. A functional vacuum booster should pull the brake pedal down as the engine starts. With that said, a vacuum booster can occasionally pass all of these field tests and still have a bad internal vacuum metering system that causes a poor brake assist complaint. Since there is no field test for this condition, the brake booster should be replaced.

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TESTING HYDRAULIC BOOSTERS

Many light truck applications use the hydraulic power steering pump to provide brake assist. Test the steering pump performance by turning the steering wheel from lock-to-lock. If the steering assist feels less than normal, use a wear gauge to measure wear in the serpentine belt.

Next, inspect the power steering fluid for level, viscosity and color. Deviations from normal viscosity or color might indicate that brake fluid or engine oil has been added to the pump reservoir.

Next, pump the brake pedal to discharge the hydraulic assist. With the brake pedal applied, the pedal should mildly “kick” or “bump” against foot pressure as the engine is started. If not, the hydraulic assist unit is worn or defective.

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FRICTION PERFORMANCE

Deficient brake friction performance is a leading cause of poor braking assist complaints. In many cases, the brake friction is of poor quality or of an incorrect formulation for the vehicle application. Semi-metallic brake friction, for example, might feel less responsive when cold than organic or ceramic friction compounds. Last, brake friction can be contaminated by rust, brake fluid leaking from calipers or wheel cylinders, or from axle lube leaking from an axle shaft oil seal. Always make sure that the brake lining is replaced with premium quality brake friction that is correct for the vehicle application.

SETTING THE HEIGHT RIGHT

Adjusting the brake pedal to the correct height is essential in resolving brake pedal assist complaints.

Adjusting the brake pedal to the correct height is essential in resolving brake pedal assist complaints.

BRAKE PEDAL SPECIFICATIONS

Brake pedal height, free play and minimum reserve distance adjustments are often forgotten when diagnosing brake pedal assist complaints. Correct brake pedal height should provide enough brake pedal travel to fully engage the brakes. Pedal height is measured on most Toyotas from the face of the brake pedal pad to the asphalt sheet under the carpet and is adjusted by loosening the clevis lock nut and turning the brake pushrod. Next, adjust the brake light switch to make sure the brake lights are off as the pedal returns to full height. The pedal height specification for our Land Cruiser ranges between 7.23 and 7.63 inches.

Brake pedal free play is the distance the brake pedal travels before engaging the brake booster. Brake pedal free play is required to completely release both the brake booster and the brake light switch. Brake pedal free play should be checked with the engine off by using the fingertips to accurately gauge resistance in brake pedal travel. On our Land Cruiser, brake pedal free play is between 0.04” to 0.240” and is adjusted by turning the brake pushrod.

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Brake pedal reserve distance on our Land Cruiser is the distance as measured from the face of the brake pedal pad to the asphalt floor pad with the engine running and 110 pounds of pressure applied against the pedal. The brake pedal reserve distance for our Land Cruiser is 4.57 inches. Excessive brake pedal reserve travel usually indicates mechanical or hydraulic problems in the brake system. If the brake pedal reserve distance exceeds specifications, check the disc brakes for loose wheel bearings or seized caliper guides. Inspect brake lining and drum for wear and replace the self-adjusters if they are worn or broken on drum brakes.

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Last, it’s always best to address excessive brake pedal travel or a poor brake assist complaint by flushing the old brake fluid from the brake system. In most cases, you’ll notice a distinct increase in brake pedal height. If brake pedal travel remains excessive, apply light pressure to the brake pedal pad. If the pedal slowly sinks to the floor, the master cylinder must be replaced.

Courtesy Import Car.

 

 

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