Brake System Inspection and Diagnosis
Understand how disc, drum and ABS brakes work, measure wear against spec, and trace a brake complaint to its cause.
Table of contents
Brakes turn a moving vehicle's energy into heat, and a failure can kill. Good brake work starts well before a wrench comes out. You need to know how the system works, measure each part against the spec, and find the real cause of a complaint. A new set of pads on a seized caliper is a comeback, and it may be a crash.
What You Will Be Able to Do
- Explain how pedal force becomes braking force at each wheel.
- Measure pads, rotors and drums, and decide whether each part passes.
- Inspect hoses, lines, calipers and drum hardware.
- Use symptoms to narrow down a brake complaint.
- Explain how ABS works and read its common fault codes.
How Hydraulic Brakes Work
Hydraulic brakes rely on Pascal's Law: pressure on a confined fluid is transmitted equally in all directions. When the driver presses the pedal, the master cylinder pushes on the fluid, and that pressure reaches every caliper and wheel cylinder at the same time. Fluid does not compress, but air does, which is why air in the lines makes the pedal feel soft.
| Part | Job |
|---|---|
| Brake booster | Multiplies pedal force, using engine vacuum, hydraulics or an electric motor |
| Master cylinder | Turns pedal force into pressure, in two separate circuits |
| Steel lines | Carry fluid along the body |
| Flexible hoses | Carry fluid to parts that move with the suspension |
| Calipers or wheel cylinders | Turn pressure into force on the pads or shoes |
| ABS unit | Controls pressure at each wheel during a hard stop |
The master cylinder has two circuits, split front and rear or diagonally. If one circuit leaks, the other still gives some braking, but the pedal travels further and the red brake warning light may come on.
Steel brake lines use double flare or ISO bubble flare fittings. Match the flare type to the vehicle, use a flare nut wrench, and never use a compression fitting on a brake line.
Disc Brakes
A disc brake squeezes a rotor between two pads. Most cars use a floating caliper, which has a piston on the inboard side and slides on pins so that the outboard pad is pulled in. A fixed caliper has pistons on both sides and does not slide.
Pad Thickness
Measure the friction material only, not the steel backing plate. Pull the wheel to see both pads, because the inboard pad cannot be judged through the wheel.
| Friction left | What it means |
|---|---|
| 10 to 12 mm | About new |
| 3 to 4 mm | Worn, recommend replacing |
| 2 mm | Replace now |
| 2/32 in (about 1.6 mm) | A figure used by some state inspections, not a shop target |
Use the vehicle maker's limit and your state's inspection rule when they differ, and go with the stricter one. A pad near its limit may wear out before the next service, so a shop will usually recommend replacing at 3 mm.
Compare the pads with each other. Even wear is good. A difference of more than about 2 mm between the inner and outer pads points to the caliper, not the pads.
| Wear pattern | Likely cause |
|---|---|
| Inner pad much thinner | Seized slide pins or a sticking piston |
| Outer pad much thinner | Stuck pad in the bracket, or a binding caliper |
| Tapered pad | Worn or corroded hardware, or a bent bracket |
| One side of the axle worn much faster | Collapsed hose, seized caliper |
Rotor Thickness and Measurement
Every rotor has a minimum thickness, stamped or cast on it, because a thin rotor cannot absorb the heat and may crack. Two figures matter:
- Discard thickness: The rotor must be replaced at or below this figure. It is often stamped on the rotor as MIN TH.
- Machine-to thickness: The thinnest a rotor may be after machining. It sits above discard so that the rotor can still wear safely in service.
Machining, also called resurfacing, cuts the rotor to restore a smooth, flat, parallel surface and remove grooves and scoring. It only makes sense if enough metal stays behind.
| Check | Tool | Typical limit |
|---|---|---|
| Thickness | Outside micrometer | Above the stamped minimum |
| Lateral runout | Dial indicator | Often 0.002 in (0.05 mm), some allow 0.003 in |
| Thickness variation | Micrometer at 6 to 8 points | Often 0.0005 in (0.013 mm) |
Lateral runout is how much the rotor face wobbles side to side as it turns. Clean the hub, seat the rotor with the lug nuts torqued, set the indicator on a fixed point with its tip on the friction surface, zero it, and turn the rotor a full circle. The reading is the total indicator reading.
Thickness variation, also called parallelism, is the change in rotor thickness around its circumference. Measure at evenly spaced points at the same distance from the edge. As the high and low spots pass the pads, they push the fluid back and forth, and the driver feels it as pedal pulsation, even when runout is in spec. Runout comes first in time, since a wobbling rotor gradually wears thick and thin spots into its faces. A clean hub prevents much of the problem.
Worked Example: Can This Rotor Be Machined?
A rotor is stamped MIN TH 26.0 mm, and its machine-to figure is 26.8 mm. You measure 27.2 mm at the thinnest point. It has scoring, and the lathe needs to take 0.3 mm from each side to clean it up.
27.2 - 0.3 - 0.3 = 26.6 mm
That is below the 26.8 mm machine-to figure, so the rotor must be replaced, even though it is still above discard.
Caliper and Slide Pin Checks
- Slide pins should move freely by hand. A seized pin causes uneven wear, drag and sometimes a pull.
- Boots must be intact. A torn piston boot lets water reach the piston, which can seize it.
- Look for wet fluid around the piston and the bleeder.
- With the vehicle raised, spin the wheel after a pedal application. It should turn with only a light drag.
Stuck or dry slide pins keep the caliper from floating, which leads to uneven pad wear and dragging brakes. Lubricate them only with silicone-based or synthetic caliper grease. Petroleum grease swells and destroys rubber boots.
Hoses and Lines
Check every hose and line, front to rear, with the wheels turned.
- Replace a hose with cracks, chafing, leaks, or a bubble or bulge. A bulge means the hose is weakened and could burst under pressure.
- A hose can also collapse inside, with no sign outside. It lets pressure in but holds it back from returning, so that brake drags and overheats. Loosen the bleeder on the dragging caliper. If the wheel suddenly frees up, the hose is the likely cause.
- Check steel lines for rust, kinks and wet spots, especially on vehicles from salt-belt states. Report any scaling rust. A rusted line can burst during a hard stop.
Drum Brakes
Drums are common on the rear of smaller cars and on many trucks. Hydraulic pressure pushes a wheel cylinder's pistons out, which forces two curved shoes against the inside of a spinning drum.
| Part | Job |
|---|---|
| Drum | Rotating friction surface |
| Shoes | Friction material on a curved steel backing |
| Wheel cylinder | Pushes the shoes outward |
| Return springs | Pull the shoes back on release |
| Self-adjuster | Keeps shoes close to the drum as they wear |
| Parking brake lever and cable | Apply the shoes mechanically |
Inspect drums with these checks:
- Linings. Check the thickness against spec, commonly at least about 1 mm above rivets or the shoe table. On duo-servo brakes, the primary and secondary shoes wear at different rates, which is normal.
- Drum. Measure the inside diameter. The maximum is stamped on the drum, and the drum must be replaced if machining would exceed it. Check for heat spots, scoring and an out-of-round shape.
- Wheel cylinder. Peel back the boot gently. Fluid inside means it is leaking, so replace it.
- Hardware. Check for tired, stretched or heat-discolored springs. Replace the hardware with the shoes.
- Self-adjuster. Check that the star wheel turns freely.
Drum brake problems have their own patterns:
| Symptom | Likely cause |
|---|---|
| Grabbing | Fluid or grease on the lining |
| Dragging | Over-adjusted shoes, weak springs, stuck cylinder |
| Low pedal that improves when pumped | Out-of-adjustment shoes |
| Pulsation felt in the seat or parking brake | Out-of-round drum |
| Squeal | Glazed shoes, dry backing plate contact points |
Diagnosing a Brake Complaint
Start by getting the details. Ask when it happens, how often, and at what speed. Road test the vehicle if it is safe to do so, then match the symptom to the likely cause and prove it with a measurement.
| Symptom | Likely cause | First check |
|---|---|---|
| Spongy or soft pedal | Air in the system, or a failing master cylinder | Bleed, then test the master cylinder |
| Pedal sinks slowly while held | Internal master cylinder leak, or external leak | Look for leaks, test the master cylinder |
| Hard pedal, poor stopping | Booster or vacuum problem | Test booster vacuum and check valve |
| Pulls to one side when braking | Uneven force from a stuck caliper, collapsed hose or contaminated pads | Compare wheel heat, inspect both sides |
| Pedal pulsation | Rotor thickness variation or runout | Measure thickness and runout |
| Squeal | Wear indicator, glazing or missing hardware | Inspect pads and shims |
| Grinding | Friction gone, metal on metal | Stop driving, inspect now |
| Burning smell, hot wheel | Dragging brake | Check slide pins, piston and hose |
| Fade after long braking | Overheated pads or boiled fluid | Check pads, fluid and how it is driven |
| Low fluid in the reservoir | Worn pads or a leak | Inspect pads, then check for leaks |
Brake fade is a loss of stopping power from heat. Long or repeated hard braking overheats the pads, and it can boil moisture in the fluid. The driver feels a hard pedal with less stopping as pads overheat, or a soft pedal if the fluid has boiled.
Low fluid deserves a closer look. As pads wear, caliper pistons move out and the fluid level drops, so a slightly low reservoir is often just worn pads. Do not simply top it up. A level below the MIN line means you must look for worn pads or a leak.
Warning: A grinding brake or a pedal that goes to the floor is a safety problem. Do not move the vehicle until it has been checked. Tell the service advisor right away.
ABS and Stability Control
An anti-lock brake system (ABS) keeps a wheel from locking during hard braking, so that the driver can still steer. It does not always shorten stopping distance, especially on loose surfaces.
- Wheel speed sensors read a toothed tone ring or a magnetic encoder at each wheel.
- The control module compares wheel speeds. A wheel that slows too quickly is about to lock.
- The hydraulic unit uses solenoid valves and a pump to hold, drop and rebuild pressure at that wheel, many times a second.
The driver feels pedal pulsing and may hear a buzz during a real ABS stop. This is normal. The same hardware usually supports traction control and stability control.
ABS Fault Diagnosis
The amber ABS light means the module has stored a fault, and the anti-lock function may be off. The red brake light is a separate warning for low fluid, a parking brake that is on, or a serious hydraulic fault. Base braking usually still works with only the amber light on, but tell the customer not to rely on ABS until it is fixed.
Faults in the module are chassis codes, which begin with C. You need a scan tool that can reach the ABS module, as a basic engine code reader may not.
Wheel speed sensor faults are the most common. On many GM vehicles, C0035, C0040, C0045 and C0050 point to the left front, right front, left rear and right rear wheel speed sensor circuits. Use the service information for the vehicle in front of you, because codes vary by maker.
- Read the code and freeze data.
- Watch live wheel speed data on a straight road test. All four wheels should read about the same speed.
- Inspect the sensor, its wiring and connector, especially where it flexes with the suspension.
- Check the tone ring for missing teeth, cracks, rust and metal debris.
- Test the sensor as the service information directs. Older passive sensors can be checked for resistance, commonly in the range of 800 to 2,000 ohms, but active sensors cannot be tested this way.
Newer Brake Systems
Check the service information before you work on these. Electric parking brakes need a scan tool or a service mode to retract the caliper. Hybrids and EVs blend regenerative braking with the hydraulic system, and some brake-by-wire systems can build pressure on their own when a door opens, which can injure your hands. Put the system in service mode first. Treat any orange high-voltage cable as off limits unless you are trained for that vehicle.
Common Mistakes
| Mistake | What it costs |
|---|---|
| Judging pads through the wheel | Missing a worn inner pad |
| Skipping the micrometer | A rotor under its limit goes back on the car |
| Replacing pads without asking why wear was uneven | Same wear, comeback |
| Topping off low fluid without checking | Missing a leak |
| Ignoring a bulged hose | Brake failure under hard use |
| Blowing out dust with air | Possible asbestos exposure for the whole bay |
Check Your Understanding
- The inner pad on the left front measures 2 mm, and the outer measures 8 mm. What do you do before fitting new pads? Find out why. Check the slide pins, piston and hose, and service the caliper first. New pads alone will wear the same way.
- After a fast road test, a car pulls left under braking, and the left front wheel is much hotter. Which parts do you check? The left front caliper and its hose. A dragging or seized caliper, or a collapsed hose, is likely.
- A rotor passes for runout, but the pedal still pulses. Which measurement is next? Thickness variation, at several points around the rotor.
- The reservoir is below MIN, and the pads are half worn. What should you suspect? A leak. Pad wear alone should not take it that low, so look at calipers, wheel cylinders, lines and the master cylinder.
- A scan shows C0045 on a GM truck, and the rear left wheel speed stays at zero on a road test. What is your next step? Inspect that sensor, its wiring and its tone ring, then test as the service information directs.
Key Takeaways
- Pascal's Law carries pedal pressure through fluid to each wheel. Air in the fluid ruins this.
- Measure pads, rotors and drums. Use the stamped limits and the maker's spec.
- Uneven wear is a symptom. Find the cause before you fit new parts.
- Runout and thickness variation both cause pulsation. Measure both.
- Low fluid means worn pads or a leak. Find out which.
- ABS codes begin with C, and wheel speed sensors are the usual cause.