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Symptom: The fluid in the reservoir is frothy with thousands of tiny air bubbles, often accompanied by heavy steering and a noisy power steering pump.  In extreme cases the fluid can be forced out of the reservoir.

Cause: Providing the system has been bleed correctly, the problem will be due to air being sucked into the system. The only areas that air can be sucked in to the system is the front seal of the power steering pump, the low pressure connection on the pump or the pipework between the pump and the reservoir. Once air has been drawn into the system it is whipped together with the power steering fluid by the vanes of the impeller in the pump creating froth. This frothy fluid cannot transmit any force so the steering becomes heavy.

Solution: It is more cost effective to check the pipework first than to change the pump. Check that the rubber hose between the reservoir and the pump inlet connection has not become hard and that the clamps are tight and are creating an airtight seal. 
On certain power steering pumps the low pressure connection is bolted to the pump and is sealed with an o-ring.  This o-ring can become hard or damaged allowing air to be drawn into the system so replacing would cure the problem.  
If after checking the low pressure side of the pump the problem persists then replacing the pump will be necessary.
Once there is frothy power steering fluid in a system it is recommended that the vehicle is left standing without the engine running for about an hour and then go through the bleeding procedure to eliminate air from the system.