Brake Pedal seems OK, Hissing Sound in cabin when pedal pressed, no brake at the end.

[gregg3gs]

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.

[ard]

Any chance leaves have clogged the drains around the brake vacuum pump and water has collected there? Just a WAG based on some E39 stuff

[gregg3gs]

Originally Posted by ard

Any chance leaves have clogged the drains around the brake vacuum pump and water has collected there? Just a WAG based on some E39 stuff

I will be checking tomorrow (once I have sun back out).

Few things so far:
- with radio off you can head hissing (like air leaking) from brake pedal in the cabin
- there are times where, when you press the brake quickly intending to sop, car takes longer to stop and you do not get a confident feeling that you are doing anything.

[pshovest]

Originally Posted by ard

Any chance leaves have clogged the drains around the brake vacuum pump and water has collected there? Just a WAG based on some E39 stuff

Sure sounds like the E39 problem I lived thru. Has X5 been outside in rain lately? To avoid any confusion, you need to check around the vacuum booster, mounted on the firewall under the cabin air filter. Check for a high water mark on booster. In my case the water would buildup but drain very slowly, so by the time I inspected, there was zero water in the space, but a visable mark on the booster.

[SeanC]

It's gonna get worse. Mine had the recall done, but brakes still failed couple of months ago. This might help:

[80stech]

I think if you have a "hissing" sound when applying brakes and no power assist the booster diaphragm is probably ruptured.

[jopecasa]

[gregg3gs]

Originally Posted by 80stech

I think if you have a "hissing" sound when applying brakes and no power assist the booster diaphragm is probably ruptured.

So today I tightened the clamp on the hose from the vacuum to the brake booster.
I drove the car and every time I press brake pedal I still hear the air escaping or hissing.
I did a booster test I found on youtube: pressed the brake pedal multiple times when car was off, held the pedal pressed, started the car and the pedal did go down a little.

When you say that diaphragm failed, would you say I need a new brake booster all together?

Is there a way to tell the pump is good, does it matter if I hear hissing sound?

Also, I did a visual inspection, the booster is not under water.

[gregg3gs]

... side note... thanks for all replies, keep them coming.

[80stech]

It is possible to replace the diaphragm in the booster but you probably will not be able to purchase the diaphragm by itself and will probably have to replace the booster as a unit ;(

I'm not sure what you mean by "pump" does E70 have a dedicated vacuum pump for the brake booster ? Regardless, the way you are describing the problem I would bet dollars to donuts the diaphragm is ruptured.