My brakes barely work and I think i have the culprit but was looking for other ideas before i throw parts at it.

When i press the brake pedal it goes to the floor before doing anything
There is pretty loud hissing when I press the brake pedal
Brakes never really build up pressure
It lost a ton of brake fluid, about 8 oz but there are no leaks anywhere that I can see.
I am getting all sorts of misfire codes
Master cylinder is newer

FCP shows SJG500030 as the part number, but that seems to be a 206-2005 RR sport booster, could they be the same?

• Power Brake Booster
  2003 Land Rover Discovery HSE Sport Utility 4.6L V8 Gas VIN:4 Automatic
  2003 Land Rover Discovery HSE Sport Utility 4.6L V8 Gas VIN:6 Automatic

From reading it sounds like the brake booster is leaking fluid into the vacuum system, into the intake which is probably causing the misfire.
Does this make sense, anyone have any other opinions, ideas? The brake booster is about the only thing I have not replaced in the system.

Thanks,

 

Unplug the vacuum line from the booster and cap/plug it. See if you codes go away. Borrow a vacuum tester from Advance or Oreilly and connect it to the booster hose. Pump up the vacuum and see if it holds. If it does not, you have a leaking booster.