Dragging Brakes
FRONT DISC BRAKESCauses
- Corrosion deposits build up inside of the caliper along the piston seal. This prevents the piston from fully releasing, and causes the pads to slightly drag on the rotor.
- Caliper slide joints/pins binding. If the slide joints/pins are binding, the caliper piston will release the inner pad but the outer pad will remain in contact with the rotor. This is often indicated by the outer pad wearing much faster than the inner pad.
Disc Brake Mechanical Force Diagram:
NOTE: Floating calipers require free and unrestricted movement on the caliper mounting. Floating or sliding calipers have pistons only on one side of the rotor. The first part of the piston's travel forces the inner pad against the rotor, then further travel forces the movable part of the caliper to pull the outer pad against the rotor.
- Pinched or internally collapsed brake hose. Hydraulic pressure from the master-cylinder forces brake fluid through the hose, but residual pressure is then trapped behind the restriction in the hose causing the brakes to drag.
- A blocked compensating port in the master-cylinder. The compensating port allows pressure to quickly equalize upon the release of the brake pedal.
- Lack of clearance between brake master-cylinder and push rod. Lack of clearance will prevent the master-cylinder piston from fully retracting in its bore. This can block the compensating port, and maintain hydraulic pressure in the brake system.
- Oil contamination of the brake fluid. Oil (or any other type of petroleum product) that has contaminated the brake fluid will cause the rubber seals in the master-cylinder and the calipers to swell. This swelling causes the caliper pistons to bind and stick.
What to Check For
1. Inspect the master-cylinder and brake fluid.
- Inspect the rubber gasket on the inside of the master-cylinder lid. If the gasket is swollen or bloated, the brake fluid is contaminated with oil.
- Siphon or scoop a small amount of brake fluid out of the master-cylinder and place into a styrofoam cup filled 2/3 of the way with water. Brake fluid will mix with the water while oil contaminants will float on the surface and dissolve the styrofoam.
NOTE: If any oil contamination is present ALL brake components containing rubber seals will need to be replaced or overhauled. This includes the master-cylinder, both front calipers, both rear wheel cylinders, and all flexible brake hoses.
- With the master-cylinder lid off, slowly depress and release the brake pedal several times. As you are releasing the pedal a steady stream of brake fluid should be visible bubbling up from the bottom of the master-cylinder. If no stream is visible siphon or remove all brake fluid from the master-cylinder and verify that the compensating port is not blocked.
WARNING: Wear eye protection and view the master-cylinder from an angle. If the brake pedal is released suddenly, brake fluid may shoot out of the top of the master-cylinder.
- Inspect for clearance between brake pedal pushrod and master-cylinder or power booster. Verify there is a small amount of clearance at pushrod, allowing master-cylinder piston to fully return in its bore, without blocking compensating port.
2. Inspect all brake lines leading from the master-cylinder to the calipers for evidence of pinching or deterioration.
NOTE: Inexperienced mechanics may sometimes clamp the flexible brake hoses with locking pliers while removing calipers. Closely examine the brake hoses for indentations or teeth marks. If any are present the hoses should be replaced. Brake hoses are double walled, damage to the interior pressure wall is not always visible.
- If the brake is locked up, opening the bleeder valve will release any residual pressure being held in the line by a faulty hose or master-cylinder. If the brake remains locked up, inspect the caliper piston and slides for seizing or binding.
3. Inspect the caliper slide joints/pins for signs of rust or corrosion, and for proper lubrication. Clean and lubricate with "high temperature" silicone grease as necessary.
4. Inspect calipers.
- Inspect for damaged or missing mounting hardware.
- Inspect piston dust boots for cuts, tears or melted appearance.
- Inspect for brake fluid leakage past piston seal or dust boot.
- Check for excessive resistance when pistons are retracted with bleeder valve open.
- Rebuild or replace calipers as necessary, if any problems are found.
5. Inspect pads and rotors for signs of overheating (bluish purple discoloration, cracks, glazed appearance, or a thin brick red oxidation layer evenly covering all brake components), replace as necessary.
REAR DRUM BRAKES
Causes
- Corrosion deposits build up inside of the wheel-cylinder along the seals. This causes the wheel-cylinder pistons to bind and prevents them from withdrawing upon release of the brake pedal.
- Improperly adjusted shoes/linings or emergency brake. If the shoes/linings or emergency brake are adjusted too tightly, the shoes will be in continuous contact with the drum.
- Stuck or frozen emergency brake cable. A corroded or pinched cable or cable housing will not allow the rear brakes to release.
- Weak or broken return springs. Proportioning valves maintain a residual pressure on the drum brake system after the brake pedal has been released. If the return springs are too weak to overcome the residual pressure in the system,the shoes will drag against the drum.
- Contact pads between the backing-plate and shoes/linings are burred, grooved, or lacking lubrication. This prevents the shoes/linings from properly returning to their initial position.
- Pinched or internally collapsed brake hose. Hydraulic pressure from the master-cylinder forces brake fluid through the hose but residual pressure is then trapped behind the restriction in the hose causing the brakes to drag.
- Blocked compensating port in the master-cylinder. The compensating port allows pressure to quickly equalize upon the release of the brake pedal. Can be caused by contaminants in the brake fluid, corrosion of the cylinder or port, or a pushrod which has been adjusted too tightly.
- Oil contamination of the brake fluid. Oil (or any other type of petroleum product) that has contaminated the brake fluid will cause the rubber seals in the master-cylinder, wheel cylinders, and calipers to swell. This swelling can cause any of these components to bind and stick.
What to Check For
1. Inspect the master-cylinder and brake fluid.
- Inspect the rubber gasket on the inside of the master-cylinder lid. If the gasket is swollen or bloated, the brake fluid is contaminated with oil.
- Siphon or scoop a small amount of brake fluid out of the master-cylinder and place into a styrofoam cup filled 2/3 of the way with water. Brake fluid will mix with the water while oil contaminants will float on the surface and dissolve the styrofoam.
NOTE: If any oil contamination is present ALL brake components containing rubber seals will need to be replaced or overhauled. This includes the master-cylinder, both front calipers, both rear wheel cylinders, and all flexible brake hoses.
- With the master-cylinder lid off, slowly depress and release the brake pedal several times. As you are releasing the pedal a steady stream of brake fluid should be visible bubbling up from the bottom of the master-cylinder. If no stream is visible siphon or remove all brake fluid from the master-cylinder and verify that the compensating port is not blocked.
WARNING: Wear eye protection and view the master-cylinder from an angle. If the brake pedal is released suddenly, brake fluid may shoot out of the top of the master-cylinder.
- Inspect for clearance between brake pedal pushrod and master-cylinder or power booster. Verify there is a small amount of clearance at pushrod, allowing master-cylinder piston to fully return in its bore, without blocking compensating port.
2. Inspect all brake lines leading from the master-cylinder to the wheel cylinders for evidence of pinching or deterioration.
NOTE: Inexperienced mechanics may sometimes clamp the flexible brake hoses with locking pliers while removing wheel cylinders. Closely examine the brake hoses for indentations or teeth marks. If any are present, the hoses should be replaced. Brake hoses are double walled, damage to the interior pressure wall is not always visible.
3. With the emergency brake released, verify that the wheels spin freely. If the wheels drag, re-adjust the shoes/linings and parking brake as necessary.
4. With the drum removed, apply and release the emergency brake several times and verify that the cable and linkage are not sticking.
5. Inspect the contact pads between the shoes and backing-plate. Remove any burrs present. Clean and lubricate with "high temperature" silicone grease.
6. Inspect all return springs. Replace springs as a set if any appear worn, overheated, or distorted.
7. Inspect wheel cylinders. Check for leakage, build up of rust or corrosion inside dust boots. Check for free movement of pistons in their cylinder bores. Pistons should not bind or stick. Rebuild or replace as necessary.
8. Inspect shoes/linings and drums for signs of overheating (bluish purple discoloration, cracks, or glazed appearance). Replace as necessary.