Why is my steering pump oil frothing?

ive brought car , noticed squealing noise when steering car. mechanic put steering oil in, oil frothed if hot or boiling. seemed fix problem- temporarily.
after 40 mins of driving steering became stiff , likes squealing screaching noise. know , whether cost me alot fix?
thanks

ive brought car , noticed squealing noise when steering car. mechanic put steering oil in, oil frothed if hot or boiling. seemed fix problem- temporarily. after 40 mins of driving steering became stiff , likes squealing screaching noise....

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