Shut off the engine. Inspect the Vacuator Valve (or scavenge line) for damage. If damaged, replace. If plugged or full of contaminant, check the pre-cleaner tubes, which should be free of contaminant. If plugged or excessive contaminant is visible, the pre-cleaner tubes will need to be cleaned.
To clean the pre-cleaner tubes, remove the housing service cover and the Vacuator Valve and leave the filter installed (to avoid dust from entering the air induction outlet). Use a low-volume of compressed air go gently blow out the separator tubes. The compressed air can be pushed through both sides of the tubes AND from the drop tube where the Vacuator Valve attaches.
If compressed air is not available or the use of compressed air was not effective due to dried contaminant within the housing, remove the air cleaner from the machine and cover the air intake pipes to prevent contaminant from entering. Remove the primary and secondary filters and the Vacuator Valve. Use a low pressure water (e.g. garden hose) to clean the tubes and inside of the housing. Direct the flow of water through the separator tubes from both ends and repeat as needed to clean out the housing. Spray out the Vacuator Valve port, altering between in and the separator tubes. Make sure that all internal housing services are dry prior to reinstalling the filters, Vacuator Valve and air cleaner on the machine.