A lumber yard reported that a Hyster S135FT diesel forklift had an extremely hard brake pedal. The operator had to stand on the pedal to stop the machine. Additionally, when the brakes were applied, the engine would lug down or stall.
The forklift uses a vacuum brake booster (hydro-boost type) to assist the driver. The vacuum is supplied by a vacuum pump driven by the engine camshaft. The technician tested the vacuum output at the pump; it was reading 0 inches of mercury (no vacuum). The vacuum pump was seized, likely due to lack of oil lubrication to the pump vanes.
However, the stalling issue pointed to a secondary fault. The brake booster was removed. The large diaphragm inside the booster was found to be torn. When the driver pressed the brake pedal, instead of vacuum assisting the push, air was rushing through the torn diaphragm into the intake manifold. This massive, un-metered air leak caused the engine to run lean and stall.
Furthermore, the seized vacuum pump had created drag on the camshaft, robbing the engine of power.
The repair involved replacing the vacuum pump with a new unit and rebuilding the brake booster with a new diaphragm kit. The hydraulic brake fluid was also flushed, as the contaminated fluid had likely caused the vacuum pump seals to fail, allowing oil to enter the pump and seize the vanes. After the repair, the brake pedal was light and responsive, and the engine RPM remained stable during braking.
