Help needed . Code 0171 on myt 2005 XJR appeared and a short time later, Restricted Performance appeared on dash. Prior to the 0171 Code I noticed the brake pedal was getting a little squishy and several days later I tested the pedal and found that it actually went to the floor with a little continuous pressure. Within a couple of days, the code 0171 first flashed. I was driving that day in Restricted Mode and while at a stop light I held the brake pedal down to the floor, probably out of frustration and within 10 seconds, the Restricted Performance notice disappeared. The light turned green and the car performed normally. The Restriced Performance will return after a few minutes.

Could the Master Cylinder be leaking from within and creating a vacuum leak?
thanks in advance

 

Well the power in power brakes is engine vacuum? So yes if the booster is leaking internally or the line from the engine has a leak that can and will set codes or make the engine run bad. And you might have a bad master cylinder too?

Maybe your lucky as the 4.2L SC engine has a known vacuum leak in that line because of the failure prone connection Jaguar used. The PB gets it's vacuum at the elbow to the SC on the passenger side of the engine.

Here is a thread about it with some good pictures.
SC Elbow Vacuum Leak

Here is the little trouble maker circled in red.





See if yours is intact or leaking.
I would chase the vacuum leaks first and then see how the MC acts.
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Thank you very much. Very much!