Brakes Grab

FRONT DISC BRAKES

Causes
- Brake rotors and pads contaminated with oil or brake fluid.
- Caliper mounting bolts loose.
- Caliper piston sticking. The piston may be sticking or becoming wedged in the caliper bore due to accumulated corrosion deposits or excessive wear. A sudden drop in pedal height followed by wheel lockup indicates a sticking piston.

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.

What to Check For
- Inspect the linings and rotors for brake fluid or grease contamination. Any brake lining which has been contaminated should be replaced. The brake rotors should be thoroughly cleaned with a commercial brake cleaner.
- Check the caliper mounting bolts and tighten as necessary.
- Check for free movement of caliper slides and pistons. Inspect piston and slide boots for cuts or tears. Look for signs of moisture contamination of slides or leakage from piston seals. Check for binding of piston when retracting into caliper bore with bleeder open. Rebuild or replace as necessary.

REAR DRUM BRAKES

Causes



- Shoes/Linings incorrectly installed. Each side should have a leading and trailing shoe. The lining surface on the trailing shoe will be slightly larger than the lining surface on the leading shoe. The trailing shoes should be installed in the rear position on each side, while the leading shoes are installed in the front position on each side. If the shoe positions are reversed, the braking performance will be very grabby and prone to wheel lockup.
- Brake linings contaminated with brake fluid or gear oil.
- Incorrect shoe size or type.
- Loose or broken brake lining.
- Brake shoe adjustment too tight or too loose.
- Weak or broken return springs.

What to Check For
- Look for the following common mistakes:
- Leading and trailing shoes are reversed.
- Both leading shoes are installed on one side while both trailing shoes are installed on the other side.
- Inspect the wheel cylinders for signs of leakage or sticking, replace or rebuild as necessary.
- Inspect the axle seals for signs of leakage. Gear oil leaking from the axle seal will usually be indicated by a spray pattern starting at the center of the drum and spreading outwards. If any signs of gear oil leakage are found, the shoes and seal should be replaced.

NOTE: Leakage at the axle seals is often caused by worn wheel bearings. If the vehicle has high mileage, it is recommended to replace the wheel bearing along with the axle seal.

- Inspect all brake springs and replace if any appear worn or distorted.
- Inspect shoes for correct fit with drum and backing-plate.
- Inspect brake lining for cracks, missing pieces or poor bonding to shoes.
- Check for proper adjustment, readjust as necessary.