Causes and Troubleshooting of Forklift Brake Faults

Forklift brake troubleshooting: The forklift traveling brake system consists of a vacuum pump, vacuum tank, vacuum booster, brake master cylinder, brake wheel cylinder, wheel brake, and brake pedal. The brake type is vacuum booster, hydraulic drive, and internal expansion front wheel brake. Common faults are as follows:


1. When stepping on the brake pedal of the forklift, it does not have a braking effect

Possible causes: There is no oil in the brake master cylinder, the brake system pipeline leaks oil, and the master cylinder cup is cracked and worn, causing oil leakage; The piston ring of the master cylinder is damaged, swollen, or excessively worn; The piston leather ring of the master cylinder is installed reversely; The piston of the slave cylinder leaks oil or the half shaft oil seal is damaged, causing oil contamination in the brake drum, causing brake failure of the brake drum and friction lining; The brake shoe friction lining is excessively worn, exposing or loosening rivets.

2. There is elastic resistance when stepping on the brake pedal of the forklift, and the braking force is insufficient

Possible cause: There is air in the braking system; Insufficient brake fluid in the master cylinder; The turbocharger oil return valve is damaged or dirty; The brake fluid cannot establish sufficient pressure to brake even if the brake pedal is pressed to the bottom; Excessive clearance between brake drum and friction lining; Loose brake shoes, etc; Improper installation of brake shoes and improper adjustment of brake clearance.

3. The brake pedal of the forklift has rebound force, and the braking effect is poor

When stepping down the brake pedal, you should listen for a "hiss" sound of intake air from the vacuum booster. If there is no such sound, you can loosen the low-pressure oil pipe of the booster, and then step on the brake pedal. If the oil pipe does not produce oil or is unable to produce oil, the oil path from the master cylinder to the vacuum booster is blocked, or the master cylinder is not working properly, otherwise the booster itself has a fault.

Possible cause: If the pipeline is blocked, remove the low-pressure oil pipe from the master cylinder to the supercharger and blow it with compressed air. If it fails to blow, the pipeline is blocked; Otherwise, the master cylinder is not working properly. The poor operation of the master pump is manifested by low oil outlet pressure (the standard is 3MPA, which can be measured with a pressure gauge), and swelling of the master pump cup and collar; The oil outlet valve is not working properly, or there is no gap between the master cylinder push rod and the piston, blocking the oil return hole, preventing the brake fluid from returning to the oil storage chamber; The leather ring of the supercharger control cylinder expands, and when the brake pedal is released, the piston cannot return quickly. When the brake pedal is continuously pressed, the pedal does not return and gradually rises, and it feels very hard when pressed down; The oil inlet hole of the supercharger control cylinder is blocked, and the atmospheric valve cannot be opened when the brake pedal is pressed, so the supercharger does not function; The sealing ring of the intake valve expands or leaks, and the brake pedal position will become higher at this time. When stepping on it, it feels very hard. When the forklift travels, the four wheels drag.

4. Brake drag, brake drum heating after long travel

Possible causes: The supercharger mechanism is stuck, and the brake stroke is improperly adjusted; The oil return hole of the brake master cylinder is blocked (covered by an inflated rubber cup or blocked by dirt in the brake fluid), resulting in poor oil return and wheel braking drag; The brake fluid is dirty and the oil return is too slow; The master cylinder cup expands, and after loosening the brake pedal, the cup cannot quickly return to its original position; The expansion of the sealing ring of the control valve causes the air valve to be loosely closed, resulting in sluggish exhaust from the air chamber in the drum membrane of the power cylinder, and the drum membrane cannot return quickly; The copper pipe between the brake master cylinder and the booster is deformed or pinched flat, causing slow oil return speed, resulting in brake drag.

5. Unsmooth braking

Possible causes: The brake drum is out of round, and the gap between the brake drum and the brake shoe is abnormal; Brake shoes not adjusted

6. It takes a lot of effort to press the brake pedal

Possible cause: The master cylinder cup or master cylinder piston ring is swollen; The copper pipe between the master cylinder and the booster is deformed or pinched flat, causing the inner hole to become smaller and the oil flow to be impeded; The copper pipe leading to the wheel cylinder is deformed, resulting in a smaller aperture and poor oil flow; Control cylinder leather ring expansion; The brake fluid deteriorates and is too viscous, resulting in poor fluidity.

7. Brake deviation

Possible causes: The braking force of the left and right wheels is uneven, causing the forklift to lean to one side when braking (check the brake clearance or the condition of the wheel cylinder on the side that grabs forward during braking; the deviation is irregular, sometimes left or right. If the braking force is first left and then right during braking, check the braking condition of the left wheel first, otherwise check the braking condition of the right wheel first); Or there is oil contamination on the brake shoe friction plate or brake drum.

8. Oil leakage (including external leakage and internal leakage) - As long as there is an oil leakage in the master cylinder, slave cylinder, booster, and pipeline, it will reduce braking efficiency or completely disable braking.

Possible causes: Oil leakage from the master cylinder and slave cylinder, mostly due to wear and tear of the cups and cups that have been used for too long; If the brake pedal feels light when pressed, it is usually due to wear and leakage of the master cylinder cup; Poor sealing of the booster cylinder piston cup, cup, or piston push rod cup, as well as the control valve cup, can cause oil leakage; When the brake shoe is too thin or the brake clearance is too large, and the brake pedal is pressed hard, the wheel cylinder cup will often turn over and cause oil leakage.

9. Excessive brake oil consumption

Possible cause: oil leakage from the pipeline; The brake spool cup in the transmission shift control valve leaks oil and enters the transmission. At this time, the smell of brake oil can be heard from the transmission oil inspection hole. Check it to confirm whether there is any brake oil leakage; Poor sealing of the piston cup, cup, or piston push rod cup of the vacuum booster booster cylinder, as well as the control valve cup, can cause excessive brake oil consumption. When the booster booster booster booster booster booster booster booster chamber is opened, the brake oil will leak out, and in severe cases, it will leak into the engine oil pan through the vacuum pump lubricating oil pipe, causing the oil to be diluted or inhaled into the cylinder and burned.

10. Vacuum booster failure

This fault can be judged by whether the supercharger intake air or not and by detecting the brake oil pressure at the brake wheel cylinder.

Possible cause: The booster operates normally, and the oil pressure at the brake wheel cylinder should reach 11~13MPa (can be detected by connecting a pressure gauge). If the pressure is lower than this value, the booster has a fault; Remove the vacuum pipe of the supercharger and block the pipe orifice with your hand. After starting the engine for a moment, if you feel that there is no suction or little suction, there is a leak in the vacuum tank and vacuum pipe, or the vacuum pump is not working properly; The working condition of the vacuum pump can be detected with a vacuum gauge. The normal vacuum value should be -0.08-0.085MPa. If it is lower than this value, the vacuum pump has a fault. Upon inspection, the vacuum pump, vacuum tank, and vacuum pipe are all normal, and the supercharger still does not work. This may be caused by a serious blockage in the air filter of the vacuum supercharger (due to the fact that the air filter of the supercharger is directly installed on the supercharger and located under the vehicle, and the working environment is harsh. This fault often occurs), or the air valve cannot be opened, resulting in the inability of the atmosphere to enter the afterburner chamber.

The braking device of a forklift includes two modes: hand braking and foot braking. There are disc type, drum type, and belt type hand brakes, and their transmission parts are mostly mechanical. There are hydraulic and pneumatic foot brakes, and the hydraulic type is divided into general hydraulic type and vacuum booster hydraulic type. Although there are various forms of foot brake, the fault phenomenon is basically the same. Here, we focus on analyzing and judging common faults of the hydraulic brake system. The hydraulic braking system of a forklift truck is mainly composed of a brake pedal, a brake master cylinder, a brake wheel cylinder, a wheel brake, and an oil pipe. Common faults mainly include: poor braking effect, sudden brake failure, brake drag, and brake deviation.