1978 brake fluid leakage

[Tbird&Amazon]

Hello,
I've recently bought a 78 Tbird and have just started to fix some issues.
It leaks a lot of brake fluid, which seems to come from between the master cylinder and the booster. Cylinder is old and rusty so I'm going to replace it. But then today, a hissing sound started as well. Only heard when the transmission is in drive/reverse, not P or neutral. Same hissing can be heard when I remove the vacuum plug from the booster, but then constantly.
So my question is basically, is this hissing just from the bad seal between cylinder and booster, or does it indicate something else wrong with the booster as well? Why does it hiss only in gear?
Brake pressure is fine, pedal won't go completely to the floor.

Since I'm a novice I wanted to check. But my first thought was to just replace the master cylinder.
Thanks and regards

 

[74 Harley]

Yes, you need to replace the master cylinder. There's no seal between the booster and master cylinder. Since your master is leaking from the back the booster is probably full of brake fluid. It would be wise to change it as well. The car is 45 years old, it would also benefit you to change all of the vacuum hoses while you are in there.

 

[74 Harley]

Need to add, the most overlooked parts of the brake system are the rubber lines. They need to be changed at least every 10 years, they deteriorate from the inside out.
You might as well settle in for new rear wheel cylinders and front calipers as well.
Very cheap peace of mind.

 

[theBSman]

Hello,
I've recently bought a 78 Tbird and have just started to fix some issues.
It leaks a lot of brake fluid, which seems to come from between the master cylinder and the booster. Cylinder is old and rusty so I'm going to replace it. But then today, a hissing sound started as well. Only heard when the transmission is in drive/reverse, not P or neutral. Same hissing can be heard when I remove the vacuum plug from the booster, but then constantly.
So my question is basically, is this hissing just from the bad seal between cylinder and booster, or does it indicate something else wrong with the booster as well? Why does it hiss only in gear?
Brake pressure is fine, pedal won't go completely to the floor.

Since I'm a novice I wanted to check. But my first thought was to just replace the master cylinder.
Thanks and regards

The hissing sound you hear is most likely the booster leaking vacuum which will usually be heard anytime brake pedal is depressed and does not necessarily have to be in gear to hear it. Press brake pedal hard when in park/neutral and you'll probably hear it. It will probably need a booster.