INTERNAL LEAKAGE.- A small amount of internal leakage is allowed to provide lubrication of moving parts. This leakage is normal and does not result in faulty operation. On the other hand, an excess of internal leakage results in slow operation, loss of power, and overheating of the hydraulic fluid. The cylinders may creep or drift and, if the leak is bad enough, the control valves may not function properly.

Internal leaks are caused by wear of the seals and mating parts during normal operation. Leakage is accelerated by using oil that has too low a viscosity because the oil thins faster at higher temperatures. High pressures also force more oil out of leaking points in the system. This is why excessive pressures can actually reduce the efficiency of the hydraulic system.

Internal leaks are hard to detect. Usually, all you can do is observe the operation of the system for signs of sluggishness, creeping, and drifting. When these signs appear, it is time to test the system and pinpoint the problem.

EXTERNAL LEAKAGE.- External leaks not only look bad but make it hazardous for the operators of the equipment. A leak that allows floor plates to become slippery may cause the operator to fall on or off the equipment and get injured. A leak that drips on hot engine parts may start a fire that could result in the loss of the equipment.

Every joint in a hydraulic system is a potential point of leakage. This is why the number of connections in a system must be kept to a minimum. Leaks often arise from hoses that deteriorate and rupture under pressure. Such a leak is usually first noticed when equipment has remained idle for a period of time and hydraulic fluid is found underneath. Figures 3-47 and 3-48 show the proper procedures for repairing hoses with reusable fittings. You can remove a medium-or high-pressure hose from its fittings by unscrewing the nipple from the socket and then the socket from the hose.

Here are some hints that will help reduce hose leakage and maintenance:

Leave a little slack in the hose between connections to allow for swelling when pressure is applied. A taut hose is likely to pull out of its fittings.

Do not loop a hose unless the manufacturer requires it. This causes unnecessary flexing of the hose as pressure changes. Angled fittings should be used instead of loops.

Do not twist a hose; twisting causes the hoses to weaken.

Use clamps or brackets to keep a hose away from moving parts or to prevent chafing when the hose flexes.

Keep hoses away from hot surfaces, such as manifold and exhaust systems. If you are unable to do so, install a heat shield to protect the hose.

Route hoses so there are no sharp bends. This is critical with high-pressure hoses.

Sometimes you can stop leaks at fittings by tightening the hose connections. Tighten them only enough to stop the leakage. If you cannot stop a leak by tightening, secure the equipment and remove the connection. Inspect the threaded and mating parts of the connector. Look for cracks in the flared ends of the tubing. If O rings are used, examine them for cuts or tears. Any damaged or defective items should be replaced.

Figure 3-47.- Replacing low-pressure hose on a reusable fitting.

Figure 3-48.- Replacement procedures for medium-and high-pressure hose reusable fittings.

Cylinders may leak around piston rods or rams. You can repair some leaks by tightening the packing located in the cylinder end cap. Tighten the end cap evenly until only a light film of oil is noticeable on the rod when it is extended. DO NOT overtighten; this results in rapid failure of the packing and causes scoring of the rod. If you find an internal seal instead of packing, the cylinder must be removed and disassembled to stop the leak. Components can leak, but care in assembly and use of new seals, packings, and gaskets during overhaul will reduce this problem.

Preventing Overheating
Heat causes hydraulic fluid to break down faster and lose its effectiveness. In many systems, heat is dissipated through the lines, the components, and the reservoir to keep the fluid fairly cool. On high-pressure, high-speed systems, oil coolers are used to dissipate the extra heat.

The following maintenance tips will help prevent overheating.

Ensure oil is at the proper level.
Remove dirt and mud from lines, reservoir, and coolers.

Repair dented and kinked lines.
Keep relief valves adjusted properly.
Do not overspeed or overload the system.
Never hold control valves in the power position too long.

If the system still overheats, refer to the manufacturer's manuals for charts that list the causes and remedies for overheating.