Forklift Brake Troubleshooting: The forklift’s travel braking system consists of a vacuum pump, vacuum reservoir, vacuum booster, brake master cylinder, brake wheel cylinders, wheel brakes, and the brake pedal. The braking system employs vacuum-assisted, hydraulically actuated, internally expanding front-wheel brakes. Common faults are as follows:

1. When the forklift brake pedal is depressed, no braking action occurs.

Possible causes include: no brake fluid in the master cylinder; oil leaks in the brake system lines; rupture or excessive wear of the master cylinder cup, leading to fluid leakage; damage, swelling, or severe wear of the master cylinder piston seal; incorrect installation of the master cylinder piston seal; oil leakage from the wheel cylinder pistons or failure of the axle shaft oil seals, resulting in oil contamination inside the brake drum and subsequent loss of braking effectiveness between the brake drum and friction linings; and excessive wear of the brake shoe friction linings, causing the rivets to become exposed, loose, or even fall out.

2. When the forklift brake pedal is depressed, there is springy resistance and insufficient braking force.

Possible causes include: air in the brake system; insufficient brake fluid in the master cylinder; a damaged or contaminated booster check valve; failure of the brake fluid to build up sufficient pressure, resulting in ineffective braking even when the brake pedal is fully depressed; excessive clearance between the brake drum and friction linings; loosening or detachment of the brake shoes; improper installation of the brake shoes; and incorrect adjustment of the brake clearance.

3. The forklift’s brake pedal has rebound, resulting in poor braking performance.

When the brake pedal is depressed, listen for a hissing sound indicative of air ingress in the vacuum booster. If no such sound is heard, loosen the low-pressure oil line to the booster and then depress the brake pedal again. If no oil flows from the line, or if the flow is weak, the fault lies in a blockage in the oil circuit between the master cylinder and the vacuum booster, or in malfunction of the master cylinder; otherwise, the issue is with the booster itself.

Possible causes: blockage in the hydraulic lines. To check, disconnect the low-pressure oil line from the master cylinder to the booster and blow it out with compressed air; if the line remains blocked, the cause is a blockage in the hydraulic circuit; otherwise, the issue lies with malfunctioning of the master cylinder. Symptoms of a faulty master cylinder include low outlet pressure (the standard is 3 MPa, which can be measured with a pressure gauge), swelling of the master cylinder’s rubber cups and seals, malfunctioning of the outlet valve, or excessive clearance between the master cylinder pushrod and piston that results in the return port being blocked, preventing brake fluid from returning to the reservoir. Swelling of the booster control cylinder’s seal can also occur, causing the piston to fail to return quickly when the brake pedal is released; with continuous application of the brake pedal, the pedal will not return and will gradually rise, feeling very stiff when pressed. If the inlet port of the booster control cylinder becomes blocked, the vent valve cannot open when the brake pedal is depressed, thereby negating the boosting effect. Additionally, swelling or leakage in the intake valve’s sealing ring can lead to a higher-than-normal brake pedal position, a stiff feel when pressing the pedal, and dragging of all four wheels during forklift travel.

4. Brake drag; the brake drums overheat after prolonged driving.

Possible causes include: jamming of supercharger components and improper adjustment of the brake travel; blockage of the brake master cylinder’s return port (either by a swollen rubber cup or by contaminants in the brake fluid), resulting in poor return flow and wheel drag; contaminated brake fluid leading to excessively slow return; swelling of the master cylinder’s rubber cup, which prevents it from quickly returning to its original position after the brake pedal is released; swelling of the control valve’s sealing ring, causing the air valve to close imperfectly and delaying venting from the air chamber in the power cylinder diaphragm, thereby preventing the diaphragm from rapidly returning to its neutral position; deformation or crushing of the copper tubing connecting the brake master cylinder to the supercharger, which slows down the return flow and leads to brake drag.

5. Unstable braking

Possible causes: the brake drum is out of round, the clearance between the brake drum and brake shoes is abnormal; the brake shoes are not properly adjusted.

6. The brake pedal is very hard to press.

Possible causes: swelling of the master cylinder cup or the master cylinder piston seal; deformation or crushing of the copper tubing between the master cylinder and the booster, resulting in a reduced inner diameter and restricted fluid flow; deformation of the copper tubing leading to the wheel cylinders, causing a reduced bore and impaired fluid flow; swelling of the control cylinder seal; or deterioration of the brake fluid, making it excessively viscous and reducing its flowability.

7. Brake Pull

Possible causes: Uneven braking force between the left and right wheels, causing the forklift to veer to one side during braking (if the front wheel on one side tends to grab first during braking, check the brake clearance or the condition of the wheel cylinder on that side; if the deviation is irregular, shifting unpredictably from left to right—e.g., veering left first and then right—inspect the left-wheel braking system first, and vice versa); or oil contamination on the brake shoe linings or brake drums.

8. Oil Leakage (including external and internal leaks) — Whether in the master cylinder, wheel cylinders, booster, or brake lines, any single point of oil leakage will result in reduced braking performance or complete loss of braking function.

Possible causes include oil leakage from the master cylinder or wheel cylinders, most often due to wear and tear on the rubber cups and seals after prolonged use. If the brake pedal feels very light when depressed, it usually indicates that the master cylinder’s rubber cup has worn out and is leaking. In addition, poor sealing at the booster’s pressure-compensating cylinder—whether due to worn rubber cups or seals on the piston, the pushrod, or the control valve—can also lead to oil leakage. Furthermore, if the brake shoes are excessively thin or the brake clearance is too large, applying firm pressure to the brake pedal can cause the wheel cylinder’s rubber cup to flip over, resulting in leakage.

9. Excessive brake fluid consumption

Possible causes: oil leakage in the piping; leakage of brake fluid from the rubber cup of the brake spool in the transmission shift-control valve, which then enters the transmission—in this case, the smell of brake fluid can be detected through the transmission oil-level inspection port, and a check will confirm whether brake fluid is leaking; poor sealing of the rubber cups, O-rings, or piston-rod seals in the booster cylinder of the vacuum booster, as well as in the control valve, can all lead to excessive brake-fluid consumption. When the booster’s pressurization chamber is opened, brake fluid will leak out; in severe cases, it may even escape through the vacuum pump’s lubricating-oil line and enter the engine oil pan, diluting the engine oil or being drawn into the cylinders and burned.

10. Vacuum booster failure

This type of fault can be diagnosed by checking whether the supercharger is drawing in air and by measuring the brake fluid pressure at the brake wheel cylinder.

Possible causes: If the booster is functioning properly, the brake wheel cylinder should maintain a hydraulic pressure of 11–13 MPa (which can be verified with a pressure gauge); if the pressure falls below this range, the booster is faulty. Remove the booster’s vacuum line and seal the open end with your hand; then briefly start the engine. If you feel no suction or only very weak suction, it indicates that there is an air leak in the vacuum reservoir or the vacuum line, or that the vacuum pump is malfunctioning. The performance of the vacuum pump can be checked with a vacuum gauge; the normal vacuum level should be –0.08 to –0.085 MPa. If the reading is lower than this, the vacuum pump is defective. If, after checking, the vacuum pump, vacuum reservoir, and vacuum lines are all found to be in good condition yet the booster still fails to operate, the likely cause is severe clogging of the vacuum booster’s air filter (since this machine’s vacuum booster air filter is mounted directly on the booster and located under the vehicle, where operating conditions are harsh, such failures occur frequently), or failure of the air valve to open, preventing atmospheric air from entering the power chamber.

The braking system of a forklift comprises two types: hand brakes and foot brakes. Hand brakes are available in disc, drum, and band configurations, with most employing mechanical actuation. Foot brakes are either hydraulic or pneumatic; hydraulic brakes, in turn, are further classified as conventional hydraulic systems and vacuum-assisted hydraulic systems. Although there are various configurations of foot brakes, the typical failure modes are essentially the same. This discussion will focus on the analysis and diagnosis of common faults in hydraulic braking systems. The hydraulic braking system of a forklift primarily consists of the brake pedal, master cylinder, wheel cylinders, wheel brakes, and brake lines. The most frequent malfunctions include poor braking performance, sudden brake failure, brake drag, and brake pull.