Codman Hakim VP Valve for normal pressure hydrocephalus

My 83-year old father was diagnosed with normal pressure hydrocephalus.  The neurosurgeon recommended to place a shunt in the spinal cord to drain out excess CSF fluid to test to see if he gets better.  After the spinal cord shunt was placed, he immediately was able to walk faster, more alert and more capable to ask / answer questions and less urinary incontinence.  By the way, the spinal cord shunt was removed after a trial of 3 days (it collected about 90 ml CSF per day)

Because my father got better with the spinal cord shunt, the surgeon placed the Codman Hakim VP  Programmable Valve surgically 2 weeks ago.  After the surgery, father was more alert and more able to ask / answer questions.  However, his walking was still very limited, just like before the surgery.  The pressure setting for the shunt is at 120 mm h2o.  Why does the spinal cord shunt improve his symptoms while the Codman Hakim VP Valve didnt improve his walking ability ?  How long would it take to see improvement with his walking ?  What should the pressure setting be at ?

I really appreciate your reply to my questions.    Thanks very much for your help.