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When using the PTO are you setting the park brake on?
Is the light coming on in the dash for the warning system?
What is the make and model of your PTO system?
I forgot to ask, is your truck auto or manual trans?
do you have the speed limiter control box on your system?
Are you comfortable with checking for power at the solenoid under the truck with the key on park brake engaged and the PTO switch on?
Have someone engage the PTO while you are under the truck with the engine off to see if the solenoid id clicking, if it is then there is a problem with the hydraulic supply from the trans, or the valve in the solenoid may be stuck.
Then it is likely the valve in the solenoid has stuck and needs to be replaced, make sure the trans oil is at the correct level and it works normal first.
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Oil and dust should not have any effect on the system, as it is designed to operate in dusty and dirty conditions.