Hydraulic System Maintenance

Hydraulic systems aren’t as complicated as you might think. Through regular maintenance you will become familiar with the components and be able to diagnose potential problems.

The components of hydraulic systems work together intimately. As a result, damage to one component may cause further damage to others. For instance, overheated oil caused by a leaky cylinder seal can break down and cause damage to other cylinders or the pump. That's why it pays to perform regular maintenance and preventative inspections to eliminate problems before they occur.

Most hydraulic systems consist of a pump, hoses and lines, cylinders and motors, valves, a cooling unit, a reservoir, filters and hydraulic fluid (oil). At the heart of the system is the pump. It uses energy from the engine to pump the fluid and create hydraulic flow and pressure. Valves control the flow of the fluid by restricting or redirecting it. Cylinders and motors are the “muscles” of hydraulic systems. Cylinders have a straight, push-pull action while motors use the energy from the fluid to turn a shaft. The speed at which these components operate is determined by the hydraulic-oil flow rate, while the hydraulic pressure determines the force they exert. The cooling unit acts to cool the fluid after it has gone through the system and the reservoir feeds the pump. Connecting all of these components are hoses, line and fittings.

A fluid ounce of prevention

The basic prescription for hydraulic maintenance is prevention. If you keep contaminants out of hydraulic systems, you will avoid nearly all common problems and failures. However, some contaminants inevitably do enter the system and scratch close-fitting surfaces in the components. Therefore, follow these guidelines to keep your hydraulic fluid in top condition.

  • Keep contaminants out of your hydraulic system. Clean the area around dipsticks, fill plugs and hydraulic filters before removing them to check or change the hydraulic fluid. Keep all fluid containers tightly sealed when stored and pour directly from the container into the system.
  • Change the fluid and filter after the initial 50 hours of use. Often, the manufacturing process allows contaminants to enter the hydraulic system. A fluid change after 50 hours will eliminate these particles. Thereafter, change hydraulic fluid and filters at regular intervals as recommended in the owner’s manual.
  • Check oil before each use. Verify that fluid levels are adequate and that the fluid is in good condition. An inadequate amount of oil can cause severe damage to pumps. If your oil appears foamy or milky, you may have a leak that is causing air to enter the system. Air will cause jerky and slow operation of the hydraulics. Locate and seal the source of any leak.

Also, air within the system holds moisture. When the system cools down after operation, the moisture can condense and mix with the hydraulic fluid. Water in the hydraulic fluid looks similar to the milky appearance resulting from air contamination. To confirm that it's water, pour 1/8 cup of hydraulic fluid into a metal can and heat it with a propane torch. If you hear popping or crackling, then the fluid has water in it and you should change it immediately.

  • Regularly check the temperature of the hydraulic fluid during operation. Is the fluid too hot to touch? Does it smell burnt? Your cooling system may be not working properly or you may have pressure-related problems. Check the hydraulic oil cooler or reservoir. They must be kept clean. Remove any dirt or other debris that inhibits airflow around them. If the fluid remains hot for extended periods, it can break down and lose its ability to lubricate adequately.


Aside from inspecting the pump for any obvious external wear and damage, you should listen for cavitation. It results when the pump does not receive the proper amount of fluid from the reservoir. Follow these steps every 50 hours or so to check for cavitation.

  1. Listen to the pump when the hydraulics are operating. If it rattles or sounds like it is full of marbles or rocks, then cavitation is occurring and the system should be shut down immediately.
  2. Check the fluid level and filter to determine if they could be limiting or restricting flow.
  3. Next, determine if any previous modifications have been made to the reservoir, inlet lines or pump. If any of the components are altered, the pump inlet flow may be affected.
  4. Check inlet lines for any leaks, bends, pinching or other discontinuities that may restrict flow. Also, many systems have inlet strainers that act as a second filter to keep damaging contaminants from reaching the pump. If your system has a strainer, regularly clean it to maintain the needed inlet flow.

Most important, maintain clean fluid and filters. Pumps have check valves, vanes, pistons and gears that can be scoured and damaged by contaminants. This will result in inefficient pumping and cause wear on other parts of the equipment.

Cylinders and motors

  • Cylinders have seals and rings that can be damaged by excess pressure and contaminants in the fluid. Once again, be sure that your hydraulic fluid is clean. Check the points where the cylinder rod moves in and out of the cylinder housing for leaks. Check the length of the cylinder rods for any dings, dents or other damages that could allow fluid and pressure to escape.

A cylinder is designed to take loads along its axis only. Side loads can decrease cylinder life by causing excess wear on seals and the rod.

  • With hydraulic motors, as with pumps, your primary maintenance consideration is clean hydraulic oil. The gears, vanes and pistons can be abraded by contaminants, resulting in pressure loss. Before each use, check the drive shaft of the motor for any damage or leakage. Be sure that motors are started and operated at the correct rpm. The owner’s manual will provide this information.


Valves consist of tight-fitting components that block or direct flow. These can be abraded and leak (internally), resulting in low hydraulic pressure. Contaminants can lodge in valves and restrict flow. Again, maintain clean hydraulic fluid to keep valves in good working order.

Other components

The next aspect of routine maintenance is checking the hoses, lines, fittings and couplers. Inspect these components to determine if they have been damaged and ensure they are in good condition and in the right location.

  • Inspect hoses and lines. Metal lines may be dented from falling tools, contact with other equipment or myriad other reasons. Inspect lines for cracking, cuts or dents that restrict flow or cause pressure leaks. Hoses can be damaged in the same manner, as well as from overheating, extended wear and defective manufacturing. Look for crimped or collapsed hoses that could be restricting flow. The hose could be internally damaged, causing pieces of the hose to enter the hydraulic system and contaminate it.
  • Check the hose routing. Sometimes brackets break and hoses and lines end up where they are not supposed to be. They can be pinched, kinked, overstretched or chafed.
  • Inspect hoses at flex points. Operate the hydraulic system and observe the flex point for kinking or stretching.
  • Check all fittings on hoses to make sure they are snug. If you notice leakage at a fitting, tighten it. Be careful to not overtighten or thread damage will result.
  • Check couplings. Always keep these points clean, they are a major source of contamination. Make sure that caps are in place when the couplings are not used and replace any caps that do not fit tightly. Before connecting any lines, thoroughly clean each coupling.
  • Keep the hoses clean by washing machinery on a regular basis. This will also help to reduce hydraulic fluid temperatures.

If a hose or line is damaged, make sure that any replacements are of the same length, size and wall thickness. Refer to the service manual for specifications.

Regular preventative inspection

It is time-consuming to thoroughly check the hydraulic system before and after each use. However, most hydraulic systems are not checked until the operator complains of poor performance. By this time, it may be too late to prevent a major repair. Aside from regular maintenance described in the owner’s manual, follow these basic procedures every 50 hours or so to determine the health of your system and prevent costly failures. Once you have located a problem, contact a professional hydraulic technician to do most repair work. They can ensure that all of the specifications and tolerances within the system are maintained.

Inspecting for restricted flow

Slow hydraulics are the result of restricted flow. Low oil level, restricted hoses and lines, and damaged valves, pumps, cylinders and motors are all possible causes.

  1. Activate the hydraulic system and determine which individual system components are operating slowly. If, for instance, the bucket on your skid-steer loader does not raise or lower at the normal speed, but other hydraulic attachments do, then the restriction is in the lines and components that service the bucket. Visually inspect these components for leaks. Pay particular attention to connecting points.
  2. If no external restriction is obvious, then the obstruction may be internal. Large contaminants may be blocking a valve or have become lodged in part of a cylinder or motor.
  3. If you cannot isolate the obstruction and the entire system is operating slowly, then check the pump. If you hear the sound of cavitation, then the pump is not getting enough flow. Determine that the fluid levels are adequate and the all filters and strainers are clean. If the problem persists, check the inlet line to make sure that there is no blockage or kinking. Also, verify that the pump shaft is operating and not damaged.

Inspecting for weak hydraulics

Weak hydraulic response is the result of low pressure. The system may be delivering the amount of flow desired, but due to pressure leaks, the system does not efficiently use the flow to perform work. Pressure problems can result from damaged pumps, worn valves and worn components in motors and cylinders. If, for example, your bucket does not lift as much weight as it should, then there is a leak in the path of flow that is allowing pressure to be lost. To check a bucket attachment, perform the following steps (or comparable steps for other components).

  1. Raise the bucket to full height and shut off the engine. Remove pressure from the line by moving the control lever back and forth once or twice. Then look carefully to see if the bucket begins to creep downward. If it does, the leak could be in the cylinder, or a valve that controls flow to the bucket, or both.
  2. Block the raised bucket and disconnect the hose that is not under pressure from the cylinder. Now, unblock the bucket. If you notice oil seeping from the port, the cylinder is leaking. If no leak is noticed at the cylinder, then a valve may be defective.
  3. Do this to all attachment points on the hydraulic system. If you cannot pinpoint the leak to a specific location, then the pump or one of the primary valves may be leaking.
  4. To verify that internal leaks are occurring, check the oil for excess heat. If internal leaks are present, oil is forced through small spaces where the leak is. This increases friction and causes the oil to heat up.

The first step in preventative maintenance is to become familiar with your hydraulic system. The key is to know where the oil flows and how each component functions. Study the layout and determine where the components are and how the hoses and lines are routed. Determine which valves service each cylinder and motor. Once you know your system, regular maintenance and preventative inspections will be easy.

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