I am getting hissing sound when I step on the brake. After doing some research, it seems like it's probably a bad diaphragm/brake booster. While inspecting the vacuum line, I found that it comes off easily from the engine intake side. Could a leaky vacuum line hose fitting (the one with the red grommet) alone be causing the hissing sound? Any help is appreciated.

 

I doubt it.

 

Your booster/servo is starting to go. Are you getting any codes?

 

no code. Just the hissing sound.

 

Okay. how is your braking in terms of stopping the rover? When I first started hearing the hissing, my brakes were okay. Over time stopping power deteriorated and i started getting codes normally associated with the MAF: 1171 & 1174. Finally it got to a point where I ran the truck over to a brake specialist who told me my booster was shot. He said it was a pretty easy fix (which it is). Before I did the booster, I did replace the vacuum line you are referring to:
Hose Brake Servo To Intake Manifold W/ Sec. Air Injec. (Genuine Part # SQB103301 ) - Land Rover hoses from Atlantic British (for vehicles with SAI).
It did not help, but it was a cheap try $22.95.
A word about the brake booster, should you decide to replace it: DO NOT BUY A REMANUFACTURED BOOSTER. Initially I installed a reman one. I had to make a panic stop when a deer tried to hump the hood of my car. One of the bolts that holds the MC on to the booster stripped, leaving me with almost no brakes. I know the OEM booster is a big ticket($540), but I realized that my son drives my car occasionally and I haul our younger daughter as well. I decided this was the one area I should not scrimp on.
in the end, new booster, great brakes, no hissing.