Brake Issue

[courtenay]

Step on brake pedal, hear hissing noise, then idle drops and gets rough. Am I looking at replacing the master cylinder, booster or both?

[Akspider]

I have a 66 Comet that did the same thing - replaced the brake booster and all was well again! There might be an external check valve that possibly could be defective, but replacing mine first didn't help. I'd go for the booster...

Roger

PS: Here's a good description on the booster function and troubleshooting: http://www.agcoauto.com/content/news/p2_articleid/129

[18Fiatsandcounting]

This sounds like a classic case of a leak in the brake booster. The booster operates off of vacuum from the intake manifold, so if the booster has a leak, you'll hear that when you step on the brake, and the idle will drop as you now have a vacuum leak on the intake.

Replacing the booster requires removing the master cylinder (suggest a flare nut wrench to remove the hydraulic line connections), and removing the 4 nuts in the drivers side footwell that secure the booster (and the steering column assembly) to the body. There may be other things that have to come out, depending on your particular model year.

One option, not necessarily the ideal one, it to plug off the vacuum line that goes from the intake manifold to the booster. You'll eliminate the vacuum leak, but you also won't have your power brakes any more. The brakes will still work fine, but the brake pedal will be much harder to push. Some people prefer that "feel", but it can be a workout for your thigh muscles.

-Bryan

[Odoyle]

As mentioned above, it sounds like a booster problem, could be internal or could just be the check valve that the hose connects too. You will have to investigate further to determine that. Best of luck.

[courtenay]

Thanks, guys. Tried running with the vacuum tube plugged, but not really happy with the feel of the brakes. Have a new booster now and am going to replace. I'm a big guy and am not looking forward to getting under the dash to disconnect/reconnect this puppy!! Assume I'm in for a full brake bleed when done (?).

[tima01864]

Pay attention to the adjustment on the booster rod.

[NSFIAT]

I’ll second that last comment - I quickly learned that the setting of the nut on my new booster did not match the setting of the nut on my old booster. Once installed it was slightly engaging the front, driver side brake causing everything to heat up and clamp down. The good news is that it can be adjusted (with patience) by disconnecting the master cylinder and pulling it forward. No need to disconnect / bleed brake lines again.

[18Fiatsandcounting]

Assume I'm in for a full brake bleed when done (?).

Yes. It's possible some enterprising person has figured out how to remove the booster without having to disconnect the master cylinder from the hydraulic lines and remove it entirely, but I just can't imagine that this would work. So, once you open up the hydraulic lines at any point in the system, the brakes should be bled.

-Bryan

[tima01864]

The booster can be a bear to remove, Thhe less in the way the better, Tight fit. Some of the more experienced could help with adjusting the booster rod.