A hydraulic hose has three parts that all need to be in good condition to run safely: an inner layer that carries fluid, a reinforcement layer (steel or fabric), and an outer protective layer. When any one of these fails, it's time to replace the hose - waiting can be dangerous.
Warning Signs Your Hose Needs Replacing
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Bubbling of the outer layer
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Abrasion of the outer layer
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Cracking or dry rotting of the outer layer
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Steel or fabric reinforcement poking through the outer layer
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Leaking around the crimp
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Weeping around the fitting flange
Leaking or weeping at the fitting flange deserves special attention - this is not something to ignore.
Never Tighten a Pressurized Line
If a fitting is leaking, don't try to tighten it while the line is under load or pressurized. A cracked fitting or bad crimp can cause the hose to fail explosively - this is how operators lose fingers, get blinded, or have oil injected under the skin, resulting in amputation. Always shut the machine off first, then tighten. Stand back when you re-pressurize the system. Wear protective clothing, gloves, and safety glasses any time you're working on a hydraulic line - the same caution applies to steel lines, since a leak there is usually a cracked flare or loose fitting.
On a job site: if it's a small drip, put a bucket under it and call for the correct hose size. If it's a bad leak, shut the truck down and change it immediately - the cost of downtime is always less than the cost of hydraulic component damage from running low on oil. And resist the urge to "just tighten it" on a drip; that can crack the fitting further and make things worse.
How to Extend Hose Life
Keep an eye on fittings, avoid placing anything on top of hoses that could pinch or crush them, and never use caustic chemicals to clean them - they erode the outer layer.
Never Undersize a Replacement
When a hose needs replacing, never install one smaller than the original. An undersized hose creates excess heat and keeps the circuit from working properly. Always match the hose to the working pressure of the circuit it's replacing.