If you dump trailer's hydraulic reservoir has made a mess and spilled hydraulic fluid, often case it's an easy fix.
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The hydraulic cylinder on your dump trailer works by pushing fluid from one end to another.
This video shows you how it works, (not affiliated with JAES)
When your dump trailer battery is low, it works incorrectly and pushes air instead of fluid, pushing the fluid out of the hydraulic cylinder and into the pump. This is what causes the hydraulic reservoir to overflow.
And that's it! That is how to fix a overflowing hydraulic reservoir.