Just looking for a little help here with my '75 280Z. I was coming home yesterday and on the last turn before my house, I pressed the brake. My petal starts going down slowly but down further than it has before and makes an audible hiss when pressed. I checked the fluid and both fluid conisters on the master cylinder were half full. I topped off and (I know silly but had no choice) drove to work this morning. It stops. Kinda. But takes me 4 times the distance to stop and makes a hiss whenever pressed.

Master cylinder?

Master Vac?

Line?

Help.

Sounds like you have a leak in the vacuum assist part of the system. Doesn't really sound like it's the master cylinder. Check the the hose running from the intake manifold to the brake booster for leaks. If you don't find any there, then it's more than likely inside the booster (the big round black thing on the firewall). I've heard those can be rebuilt but it'd be easier to just get another one and swap it.

Yeah a few guys on www.zcar.com told me the same thing. I'll check that. Is it difficult to change out?

I'd guess master-vac too. Can you pop the hood & listen, or have a helper do so, so that you can pinpoint where the sound comes from?

I popped the hood and could not really tell much, although I can hear it best inside the car, mostly while it is running.

if its while running, i'd lean even more towards the master-vac, or one of the vacuum lines, since thats only when your creating vacuum. if it were just hydraulic in nature, it should be the same running or not..HTH

Mucho thanks will check this this evening.

Since you can hear it more on the inside than under the hood, I would suspect the master-vac. It may be leaking past the plunger from the brake pedal assembly and through the firewall.

Anothe thing to check is the vacuum line check valve in the middle of the line to the master-vac.