After starting the engine, the control device and bulldozing blade failed to respond. Inspecting the hydraulic oil tank revealed it was empty. The driver confirmed filling the tank before the previous workday ended. Further examination of the engine oil pan showed an increased oil level. Removing the working oil pump for inspection uncovered a damaged rotating oil seal. Because the hydraulic oil tank sits higher, oil had seeped into the diesel engine’s oil pan through the faulty seal overnight.In response to this issue, it is necessary to replace the damaged oil seal and engine oil, refill the hydraulic oil, and purge all air from the hydraulic system to restore normal operation of the bulldozer.