**Abstract:** This paper provides an overview of the common causes of hydraulic brake system failures, such as reduced braking efficiency, complete brake failure, and uneven braking. It also outlines diagnostic procedures and effective solutions to address these issues. Hydraulic brake failures are frequently encountered in real-world driving scenarios, and this article aims to offer a detailed analysis of the most common faults for reference.
**1. Brake Drag (Brake Bite)**
**Symptoms:** When pressing the brake pedal, it feels high and stiff, or there is no free travel. The vehicle may be difficult to start or drive smoothly.
**Causes:**
(1) Rust on the brake pedal shaft makes it hard to move.
(2) A weak or broken return spring in the master cylinder piston, or swelling of the cup blocking the return oil passage.
(3) Swollen master cylinder cups, aprons, or contaminated/deformed pistons.
(4) Brake shoes that rotate freely around the support pin.
(5) Too small clearance between the brake shoe friction material and the drum.
(6) Weak or broken return springs on the brake shoes.
(7) Contaminated or highly viscous brake fluid that hinders oil return.
(8) Swollen pump cups, deformed pistons, or dirt lodged in the system.
(9) Concave or blocked brake lines that prevent proper oil return.
(10) No free travel on the brake pedal, or a damaged/weak return spring.
**Diagnosis:**
If the brakes remain engaged after releasing the pedal, it indicates drag. If all brake drums are overheated, the master cylinder may be faulty. If only one drum is hot, the problem is likely with that wheel’s brake. To diagnose, check the pedal's free travel first. If it’s missing, the push rod-to-piston clearance might be too tight. If the pedal returns slowly, inspect the brake fluid for contamination. If the fluid is clean, check the master cylinder components for swelling or faulty springs. For individual wheel issues, bleed the brake line and observe if fluid flows freely. If not, the line might be clogged. If the wheels still don’t turn, check the shoe-to-drum clearance and the return springs.
**2. Poor Braking Performance**
**Symptoms:** The vehicle decelerates slowly, and the stopping distance increases when braking.
**Causes:**
(1) Air in the brake lines.
(2) Faulty brake components.
(3) Malfunctioning sub-pump.
(4) Defective master cylinder.
**Diagnosis:**
Evaluate the brake pedal height, feel, and stability during braking.
(1) If the pedal is low and braking improves after multiple presses, the clearance between the drum and shoe or the push rod and piston is likely too large.
(2) If the pedal feels soft after a few presses, the reservoir plug’s vent hole might be blocked.
(3) If the pedal doesn’t return properly, the master cylinder cup could be ruptured, or the return spring is too weak.
(4) If the pedal goes all the way down without resistance, the master cylinder may be out of fluid.
(5) If the pedal feels spongy and bounces back, air has entered the system.
(6) If the pedal gradually sinks, check for leaks in the lines, seals, or valves.
(7) If the pedal feels hard but braking is poor, check the shoe-to-drum clearance, drum surface condition, and spring stiffness.
**3. Sudden Brake Failure**
**Symptoms:** While driving, the brake pedal is pressed fully, but the brakes suddenly fail.
**Causes:**
(1) Severely damaged brake lines or loose connections.
(2) Broken or collapsed master cylinder cup.
(3) Damaged master cylinder.
(4) No brake fluid in the master cylinder.
**Diagnosis:**
Immediately stop the vehicle and check for fluid leaks. If fluid is leaking from the master cylinder, the cup may be damaged. If fluid is leaking from the wheel cylinders, the cups may be ruptured. If no leakage is found, check the master cylinder reservoir for sufficient fluid. If the fluid level is low, the system may be empty or have internal damage.
**4. Brake Pull (Unilateral Braking)**
**Symptoms:** The vehicle veers to one side during braking.
**Causes:**
(1) Frame deformation, front axle misalignment, uneven tire pressure, or worn suspension components.
(2) Uneven clearance or contact area between the brake shoes and drums.
(3) Different wear on the eccentric sleeves of the support pins.
(4) Issues with the rear wheels similar to those on the front.
(5) Excessive friction on one side of the front wheel.
(6) Air in the system, aging hoses, or uneven brake drums.
**Diagnosis:**
Perform a road test to identify which wheels have poor braking performance. Check for leaks, tire pressure, and drum-to-shoe clearance. If everything seems normal, bleed the system to remove air. If the issue persists, inspect the master cylinder and other components. If no fault is found in the brake system, check the frame, front axle, and steering mechanism. Using a brake test bench can help determine which wheel has reduced braking force.
**Special Note on Santana Vehicles:**
Santana vehicles use a hydraulic brake system with caliper mechanisms. In case of failure, check the pedal free travel, brake fluid level, brake pad thickness, and the pressure regulator. These steps should help identify and resolve most issues.
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