I had a leak in the brake line on the rear reservoir on the master cylinder to the brake indicator assembly. A new master cylinder was installed months ago. Once I installed a new brake line with new fittings the leak was fixed. I refilled the reservoir and started the car up and pumped the brake to make sure nothing was leaking. At this point I didn’t bleed the brakes yet. Everything looked good and then all of a sudden I started to hear a hissing noise and a surge in the RPM’s. So now every time I push on the brake pedal, l get that surge of RPM’s. Seems odd that my brake booster would all of a sudden go bad. At this point I installed new vacuum hoses and a new check valve. I made sure the check valve was facing the right direction. I was hoping that would fix the problem. It didn’t. I did a vacuum check on the booster and it held. No leak. Do you think I need a new brake booster? Would bleeding the brakes fix everything or are these issues not related? My car is a 72 240z.

 Thanks 

Hissing sounds like a vacuum leak somewhere.  Surprized you can hear it in the cabin.  Does it feel like you get any assist on the braking?

Doesn’t feel like I get any assist. If I start to break while I’m still in gear, I really have to push hard on the brake to slow down and take it out of gear or the car wants to surge forward. 

There is a method for checking the function of your brake booster while on the car.  Do a search in this forum and scout out videos.  My situation is the my boosters are not now on a car so I have to try a different process.

It's likely that your booster diaphragm failed and you are hearing the hiss because it's coming into the cabin along the shaft that's attached to your brake pedal.

I don't quite understand why the engine would race, however.  I assume you have SU carbs and are taking vacuum off that manifold.

The blow out booster is probably providing more air, so the engine speeds up.

I suspect the master had more travel prior to bleeding and it was too much for a marginal booster

Thanks for your responses. I appreciate it. I’ll go ahead and get my booster replaced. 

@lcord, just a heads up some members had issues with the replacement booster they got from their local parts store.

I sent mine out to Power Brake Exchange in San Jose California to be rebuilt, if you go that route, have them powder coat it too, it will come back to you mint.

 

Edited October 2, 20213 yr by heyitsrama

Thanks so much. I’ll do that.