SAT:
2008 X5 4.8 170K Miles on clock

This is sporadic:
- I drive hard and come to quick stop... all of a sudden no brake and pedal hard
- next time I press brake pedal all is good
- I get hissing sound in the cabin when pressing brake (almost all the time)


All signs point at Vacuum Pump or bad line.

What is the best way to test this?





UPDATE 12/21/2016 - Seems Fixed (testing now)
So, an UPDATE....

- got INDY to test my vacuum pump and brake booster
- inspection indicated pump leaking
- visual inspection indicated the pump has been leaking oil, well, for a while

I purchased new pump through a friends shop and installed it last night.
At that time I also replaced Oil pressure sensor that is located under the vacuum pump.

All seems to be better now..... or so it briefly did.


SAV still does not stop as quickly as I would like and finally the system does not appear to be in tip top working order.
I just purchased new hose kit from Pump to Brake Servo


FINAL UPDATE:

The hissing sound was the Brake Booster leaking air.
The repair required replacement of the Brake Booster and I ended up flushing the brake fluid afterwards.