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Troubleshooting Article Archive:  January 2004          
Hydraulic Systems Need Venting to Prevent Damage
The hydraulic system on your tow truck is designed to have an air vent so pressure cannot build up in the hydraulic oil tank.  The manufacturer usually installs a vented cap on the tank filler tube.  The hydraulic system needs a vent because the oil level in the tank must be free to go up or down as oil comes back from and goes out to the hydraulic cylinders when you operate the tow truck bed.  When someone defeats the vent, they usually do so without intending so.  The results can range from minor loss of hydraulic oil to destruction of the hydraulic pump, reservoir and hoses.

Last year, a car carrier was in my shop for repair because the suction hose had blown off the hydraulic pump and all the oil in the hydraulic system was lost.  Pressure inflated the steel hydraulic tank like a water balloon so its sides were rounded instead of flat.  Because the operator had continued to operate the equipment even after becoming aware of the spreading lake of oil under his truck, the pump completely burned out from running dry.

I looked closely at the tank and noticed that someone had replaced the original vented filler cap with an otherwise identical non-vented plumbing cap you can buy at any hardware store.  They had filled the tank with oil, installed their new non-vented cap super-tight with a pipe wrench, and then operated the car carrier.

What's a smart solution to this problem?  Drill a vent hole in the filler neck, not in the cap.  It doesn't cost you any more money or time to drill the hole in the filler neck, which is a permanent part of the tank, rather than in the cap.

Making the vent in the neck will positively determine the direction of overflow, rather than leaving the direction up to the vagaries of cap tightening.  By drilling your vent hole pointing inward toward the truck,  you can save friendly bystanders a warm, high-pressure oil shower when someone's overfilled the tank.