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DIY Auto Service; Hydraulic Brake System Diagnosis and Repair

Updated on May 30, 2014

Hydraulic Brake System Malfucntions

A variety of problems can occur with hydraulic brake systems.

Dual Hydraulic Master Cylinder

The dual master cylinder (MC) is bolted to the vacuum booster (in most cases). The reservoir on top stores the brake fluid. Two or four lines, depending on system, are routed away from the MC
The dual master cylinder (MC) is bolted to the vacuum booster (in most cases). The reservoir on top stores the brake fluid. Two or four lines, depending on system, are routed away from the MC

Caution

If Engine, Transmission or Power Steering oils are added to the Master Cylinder Reservoir by mistake, the brake system will be contaminated and cause the seals to SWELL and fail. Every component in the brake system with rubber seals will need to be replaced.

Hydraulic Brake System Diagnosis

In order to diagnose a hydraulic problem in the brake system, you must know what a good system feels like when the brake pedal is stepped on to apply the brakes. How far does the pedal move before it is solid? Is the pedal solid or does it feel spongy? Does the pedal travel further than normal and the brake warning light comes on? Is the pedal travel affected by the temperature of the brake fluid?

If the pedal travels further than normal but has a solid feel:

  • Drum brakes may be out of adjustment.

  • Disc brake pads may be pushed back too far from the rotor, due to excessive wheel bearing endplay or rotor run-out.

If the pedal feels mushy or spongy:

  • The fluid may be contaminated with air. Remember air is compressible.

If it feels like the pedal goes almost to the floor and the brake warning light is on:

  • More than likely, one part of the hydraulic system has a leak or is full of air. Look for fluid leaks at lines, valves, wheel cylinders or calipers.

The brake pedal feels mushy or spongy only after driving a distance:

  • Moisture in the brake system is getting hot enough to boil. Steam is a gas and is compressible. Flush the brake system. Check that the brakes are not dragging.

  • The brake lines may run too close to the hot exhaust and the brake fluid is boiling. Boiled brake fluid is a gas and is compressible. This is especially true today with the temperatures of the catalytic converter exhaust systems today. Look for missing heat shields or insulate the lines from the heat.

The brake pedal slowly sinks to the floor:

  • One of the primary seals in the master cylinder is leaking fluid back to the reservoir when the brake pedal is pressed. The master cylinder needs to be repaired or replaced.

Pulsations in the brake pedal during stopping:

  • Pulsation are caused by an uneven or warped rotor surface or out of round drum brake drum. The rotors and drums can be measured to find the problem and either turned or replaced depending on the amount of wear and damage


Master Cylinder Problems

The Master Cylinder can fail hydraulically from either the master cylinder itself or a leak in the brake system. Since this system is divided into two separate hydraulic systems, only half will be affected.

  • If the Primary Piston loses fluid, when the piston is moved forward it will not make the hydraulic link between it and the Secondary Piston. The short rod on the front of the Primary Piston will mechanically push on the Secondary Piston forward. The pedal will travel further than normal, but the Secondary Piston will still create the pressure for its section of the hydraulic system. The spring will return the primary piston back to its starting point.

  • If the Secondary Piston loses fluid, the movement of the brake will still create the hydraulic link between the Primary and Secondary Piston. The Secondary Piston will move all the way forward and bottom out on the rod on front of the piston. The Primary Piston can now create pressure in the Primary portion of the system. The spring on the front of the Secondary Piston will return the Secondary Piston to the rest position.

Pumping the Master Cylinder allows more fluid into the pumping section of both pistons. This is accomplished by the forward flexing motion of the primary seals. As the seal flexes it allows fluid from the Vent port to bypass the seal and fill the cavity in front of the pistons. Small holes drilled in the pistons behind the primary also allow fluid from the Replenishing port to flow as well. As long as the reservoir has fluid, no air is allowed in. If the reservoir runs low, air could be pumped into the hydraulic section and cause problems, such as a “Spongy Pedal”.

Master Cylinder Replacement

Most vehicles use a Master Cylinder that can be replaced. Some vehicles use a master Cylinder that is part of the ABS and can only be replaced as an assembly.

To remove a common Master Cylinder:

  1. Disconnect any electrical connectors from the master cylinder.
  2. Using the proper size Tubing Wrench, loosen the lines to the master cylinder.
  3. Loosen the mounting bolts at the rear of the master cylinder.
  4. Remove the lines and mounting bolts and remove the master cylinder.

Caution: Do not get any brake fluid on painted surfaces as this will eat through the paint and plastic.

Bench Bleeding

The Master Cylinder should be bench (off car) bled before it is installed. Fill the reservoirs and slowly pump the pistons with a screwdriver while using the bleeder adapters that usually come in the box. While pumping watch for when the bubbles stop coming out of the hoses submerged in the brake fluid reservoir. The master cylinder is “Bench Bled” and ready to install.

To install a common Master Cylinder:

  1. Install the mounting bolts and torque to specifications.
  2. Install the lines loosely.
  3. Have someone push the pedal down and then tighten the lines.
  4. Install the electrical connectors.
  5. Fill the reservoir to the "Full" mark. (Don't overfill)
  6. Pump the brake pedal and see how it feels.

Note: Depending on the problem, the rest of the brake system may not have to be bled at this point.

Pressure and Vacuum Brake Bleeding Tools

The pressure brake bleeder is the most recommended but, due to the number of adapters needed the vacuum type or the Two-Man methods are used most often.
The pressure brake bleeder is the most recommended but, due to the number of adapters needed the vacuum type or the Two-Man methods are used most often. | Source

Remote ABS Unit

Some ABS units are part of the Master Cylinder and others (shown) are separate units connected to the Master Cylinder by lines.
Some ABS units are part of the Master Cylinder and others (shown) are separate units connected to the Master Cylinder by lines.

Brake Hydraulic System Types and Bleeding

The layout of the hydraulic system will determine the bleeding order. The most common method is using the farthest to closest pattern. Start at the farthest bleeder screw and work towards the closest bleeder to the master cylinder.

Caution: On vehicles with Anti-lock Brake Systems (ABS), special procedures may be required to bleed the system. Caution, some systems use a charged accumulator that stores hydraulic pressure that will need to be depleted before opening the system. Check the service information to be sure.

Front Rear Split Systems are popular on rear wheel drive cars, light and medium duty trucks. This split means that one part of the master cylinder operates the front brakes and the other section operates the rear brakes. Since the master cylinder is on the left side of the engine compartment on North American products. The general rule is the farthest brake bleeder is in the right rear.

To bleed this system:

  1. Fill the reservoir with the correct brake fluid.

  2. Using one of the three bleeding methods, open the Right Rear bleeder screw. Open and close this screw until clean and air free fluid is coming out (No Bubbles). Check the fluid level as necessary.

  3. Move to the Left Rear bleeder screw and perform the same operation.

  4. Move to the Right Front bleeder screw and perform the same operation.

  5. Move to the Left Front bleeder screw and perform the same operation.

  6. Recheck the fluid in the reservoir and don’t overfill. Clean the areas that have brake fluid on them so as not to look like a leak. Don’t get any brake fluid on the painted surfaces.

The X System is used mostly on front wheel drive cars. Since they have about 90% of their brakes in the front, a front rear split would not give the vehicle enough brakes, if the front hydraulics failed. To split the brake system 50/50 the right front brake is in the same hydraulic circuit as the left rear. The left front brake is in the same circuit as the right rear. The connected wheels form an “X” in the car. There are typically 4 lines coming off of the master cylinder.

To bleed this system:

  1. Fill the reservoir with the correct brake fluid.

  2. Using one of the three bleeding methods, open the Right Rear bleeder screw. Open and close this screw until clean and air free fluid is coming out. Check the fluid level as necessary.

  3. Move to the Left Front bleeder screw and perform the same operation.

  4. Move to the Left Rear bleeder screw and perform the same operation.

  5. Move to the Right Front bleeder screw and perform the same operation.

  6. Recheck the fluid in the reservoir and don’t overfill. Clean the areas that have brake fluid on them so as not to look like a leak. Don’t get any brake fluid on the painted surfaces.

Disc Brake Caliper Line and Bleeder Locations

To be able to bleed a disc brake system the bleeder screw need to be at the top and the line at the bottom. Switching the left and right calipers will reverse these positions.
To be able to bleed a disc brake system the bleeder screw need to be at the top and the line at the bottom. Switching the left and right calipers will reverse these positions.

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