Power Steering Reservoir

The power steering reservoir is a storage container for hydraulic fluid within a hydraulic power steering (HPS) system. While it may look like a simple plastic tank, it plays a vital role in the hydraulic circuit by handling fluid storage, deaeration, filtration, and thermal compensation.

Core Functions 

Fluid storage: Provides an ample supply of hydraulic fluid to satisfy the operating demands of the steering rack and power steering pump under all driving conditions.

Buffering and heat dissipation: Hydraulic fluid heats up as it circulates through the system; the reservoir acts as an expansion buffer to help dissipate this heat.

Air separation (anti-foaming): Returning steering fluid tends to aerate and form bubbles. Internal baffles inside the reservoir allow trapped air to rise and escape, preventing bubbles from being drawn into the pump and causing loud whining noises.

Filtration and debris trapping: In many premium vehicles, a fine micron-mesh filter is built into the base of the reservoir to catch metal shavings and contaminants, stopping them from recirculating into the pump.

Key Maintenance and Diagnostic Points

Fluid level check (most crucial): 

Cold engine check: With the engine switched off and at ambient temperature, verify that the fluid level sits between the MIN and MAX markings.

Hot engine check: Certain carmakers specify checking the level when the system is warm and running. Check the reservoir cap or dipstick markings (which often show separate HOT and COLD scales). 

Inspect fluid condition:

Fresh power steering fluid is typically clear red or translucent amber. If the fluid appears murky, turns black, or emits a burnt odour, the system has suffered severe internal wear or overheating, and a fluid flush is needed immediately.

Clogged filter mesh: 

If you experience steering whine or heaviness despite a normal fluid level, the fine mesh at the bottom of the reservoir may be choked with sludge, starving the pump. In this scenario, a fluid flush alone is not enough—replacing the entire reservoir assembly is usually necessary. 

Fluid leaks:

Inspect the hose connections at the bottom of the reservoir (suction and return lines) for leaks. Continuous exposure to engine bay heat and vibrations causes these rubber hoses to harden, crack, and seep over time.

Daily Inspection and Usage Tips

Keep it clean: Always wipe away dirt and grime around the cap before unscrewing it. Any dirt that falls into the reservoir will circulate straight into the pump, leading to premature component failure. 

Check the seal: Ensure the rubber O-ring or seal under the cap is in good condition to keep the system airtight and prevent moisture ingress, which drastically degrades fluid performance. 

Do not overfill: Never fill past the MAX line. Hydraulic fluid expands under operating heat; overfilling will cause fluid to spill out from the breather vent, dirtying the engine bay. 

System Note: Not Applicable to EPS Models

If your car is fitted with modern Electric Power Steering (EPS), it does not have a power steering reservoir. Any transparent tanks you see under the bonnet are likely the Brake Fluid Reservoir or Coolant Expansion Tank. Always confirm before topping up to prevent adding fluids into the wrong system!