If you use any kind of truck mounted crane, you will know how the hydraulics work. A hydraulic pump places downward force on one piston, which transmits force to a second piston. The force is transmitted through an incompressible liquid - usually oil. The efficiency and smoothness of operation that hydraulic systems are known for depend on clean oil.
If dirty oil, dust or debris enters your hydraulic components during repair or servicing, the longevity and performance of those components may be affected.
Evaluating Your Repair Facility
The article in Machinery Lubrication discussed the things you should look for in a good repair facility. Here is a brief summary: 
Look for signs that the shop is clean and organized. Any sign of disarray may indicate that work is not done to your standards.
The workbenches where repairs are done should be far from tools that throw off lots of dust and debris, like lathes, sandblasters, grinders, welders and torches.
Clay-based absorbents, used to keep floors dry, can also generate lots of dust. A shop should use absorbent pads to take care of spills.
Solvent tanks are used to clean parts. Check that the kind of solvent used is compatible with the seals on your equipment. Also, verify that the solvent tank is filtered and cleaned regularly to ensure that dust and debris do not get onto the equipment being cleaned.
Compressed air is used throughout a repair shop, but it can blow lots of dust around. The shop should use an air source that is filtered, dried and lubricated.
Dust and debris can get stuck in the grease used for lubrication if the grease is left open. Take a look around to see how tidy the lubrication area is and check for open lubricants.
Seals are vital to proper functioning of your equipment. Old seals can dry out or crack. Be sure that seals are stored properly in sealed bags.
The testing area is a potential hazard to your equipment if the oil used in testing is not compatible with yours or if it is not clean. Ensure that the shop is using compatible oil in its testing processes and that regular filtration of the testing oil takes place.
Because the potential for contamination is so great, it is worth the time to inspect a repair shop before sending parts for servicing there.

How Hydraulics Work
Time: 2019-08-12