Brake booster hiss?

[D Cluley]

I can hear air hissing under the dash when I apply the brakes, which I think is normal. However when I am stopped with the brakes applied, it continues to hiss the same until I let up the pedal. Is that normal, or a sign of a failing booster?

1965 Chrysler with power drums.

 

[Davea Lux]

I can hear air hissing under the dash when I apply the brakes, which I think is normal. However when I am stopped with the brakes applied, it continues to hiss the same until I let up the pedal. Is that normal, or a sign of a failing booster?

1965 Chrysler with power drums.

You probably have an air leak in the booster diaphragm. Turn off the car and test to see if the booster retains vacuum when you push down on the brake pedal. There should be enough vacuum to apply the brakes and still get an assist at least once shortly after the engine is turned off. Also, if you hear hissing with the engine off and the pedal at rest, you have a leak.

Dave

 

[Dobalovr]

Booster has a vacuum leak. Do you notice a change in idle when you press the brakes? My 70 did this and it was the booster. Being a 65 and if it has the single pot master might be a good time to goto a dual master.

 

[D Cluley]

I think the idle drops a little, but I don't remember for sure. Haven't had it out since last fall, but we're finally getting some spring here.

This booster, a dual res mc & all new lines went in about 15 years ago, but I've never been totally happy with the brakes. It stops just fine but seems like more pedal effort than a new car, and everything I've heard is how these were overboosted and touchy.
The booster does work, but I wonder if it has never been 100% right.

 

[detmatt]

Doesn’t matter now, you need a new one.

 

[Davea Lux]

I think the idle drops a little, but I don't remember for sure. Haven't had it out since last fall, but we're finally getting some spring here.

This booster, a dual res mc & all new lines went in about 15 years ago, but I've never been totally happy with the brakes. It stops just fine but seems like more pedal effort than a new car, and everything I've heard is how these were overboosted and touchy.
The booster does work, but I wonder if it has never been 100% right.

While you are at replacing the booster, be sure to check the back of the master cylinder for leaks. Fluid leaking into the booster is the main reason that they fail.

Dave

 

[Snotty]

Sometimes it's the device that the power brake vacuum hose attaches to that's broken, and not the booster. (Can't recall the name.) I've had it happen twice. Even though it's in the engine bay you can hear it inside the car. Before you replace the booster, check that valve!

 

[Davea Lux]

Sometimes it's the device that the power brake vacuum hose attaches to that's broken, and not the booster. (Can't recall the name.) I've had it happen twice. Even though it's in the engine bay you can hear it inside the car. Before you replace the booster, check that valve!View attachment 181173

Booster check valve is the part. Sometimes the cap blows off of the small fitting if the car happens to fart back thru the carb.

Dave

 

[Snotty]

Thanks. As you can see from my pic, mine is not correct, but it gets the job done.

As I said, I've lost two of them; one on this car and another on the '62 I owned. Both times the stem was cracked at the base. Put my foot on the brake and the motor would stall. I thought my booster was shot; was glad to have been wrong - both times!

 

[D Cluley]

Check, I will check the check valve.
(daily reminder that English is a weird language. )

That would be nicer than having to replace the whole booster.

 

[D Cluley]

Thanks to all.

Old booster was dropping vacuum about 5 inches when applied.

New booster is in,and feels much better, and it idles much better at stoplights.

 

[imperigal]

What would the procedure be for testing the check valve? (73 T&C) have the same hissing when stepping on the brake, accompanied by a rougher idle. Release the brake and it smoothes out again. I realize it's probably the booster, but I'm entitled to a few moments of wishful thinking Having a new booster would, however, give me a lot more confidence in stopping!

 

[Davea Lux]

What would the procedure be for testing the check valve? (73 T&C) have the same hissing when stepping on the brake, accompanied by a rougher idle. Release the brake and it smoothes out again. I realize it's probably the booster, but I'm entitled to a few moments of wishful thinking Having a new booster would, however, give me a lot more confidence in stopping!

The hissing is caused by a ruptured diaphragm. The check valve won't cause the symptoms you are describing. The check valve should make the booster hold vacuum after the engine is shut off. If the check valve is bad, the vacuum will vent back into the intake manifold and the pedal will be hard immediately after engine shut off. If your diaphragm is bad, it will leak out the vacuum after engine shut down with pretty much the same effect.

Dave

 

[Davea Lux]

What would the procedure be for testing the check valve? (73 T&C) have the same hissing when stepping on the brake, accompanied by a rougher idle. Release the brake and it smoothes out again. I realize it's probably the booster, but I'm entitled to a few moments of wishful thinking Having a new booster would, however, give me a lot more confidence in stopping!

When you get around to changing the brake booster, be sure to install a new master cylinder as well. Usually the reason boosters fail is because the master cylinder has been leaking brake fluid into the booster. Brake fluid hardens the rubber parts and rusts the metal parts on the inside of the booster causing it to go south.

Dave

 

[Snotty]

The hissing is caused by a ruptured diaphragm. The check valve won't cause the symptoms you are describing. The check valve should make the booster hold vacuum after the engine is shut off. If the check valve is bad, the vacuum will vent back into the intake manifold and the pedal will be hard immediately after engine shut off. If your diaphragm is bad, it will leak out the vacuum after engine shut down with pretty much the same effect.

Dave

I disagree Dave. I had a hissing and it was clearly from the valve. When I replaced it there was no further problem.

 

[imperigal]

I disagree Dave. I had a hissing and it was clearly from the valve. When I replaced it there was no further problem.

Did it only hiss when you stepped on the brake, accompanied by a rougher idle? I haven't done the other FSM troubleshooting steps, such as measuring pedal travel, etc.

 

[Snotty]

Did it only hiss when you stepped on the brake, accompanied by a rougher idle? I haven't done the other FSM troubleshooting steps, such as measuring pedal travel, etc.

Yes. As long as I were driving all was fine.

Dave is not incorrect about a leaking in the bladder, I've experienced that as well. My point is might be something a little easier to fix hat removing the booster. Check that valve first.

 

[Davea Lux]

I disagree Dave. I had a hissing and it was clearly from the valve. When I replaced it there was no further problem.

Check valves can and do go bad. If the check valve is bad it may hiss all the time and there will be no vacuum reserve in the booster at engine shut down. Most of the time a check valve will blow out from the engine farting back thru the the vacuum line to the valve. This is different than the hiss one gets from a leaking/ruptured diaphragm which happens when the pedal is held down. Snotty is right, the booster is a PIA to change, so you won't be out much to replace the check valve first. Most of the time if you still have an OEM booster that is 40+ years old, it will probably be bad.

Dave

 

[imperigal]

Alright. Had the brake booster rebuilt. Installed new master cylinder. Installed new rotors, calipers and pads. STILL HAVE SOFT BRAKES! Brake light will even come on. Have bled them at all 4 corners. I bench bled the MC, but then let it sit on the bench over night with the passages plugged (nothing leaked out). Could it be that I have to bench bleed it again? I am having a hard time figuring this out...never had this issue before!

 

[Davea Lux]

Alright. Had the brake booster rebuilt. Installed new master cylinder. Installed new rotors, calipers and pads. STILL HAVE SOFT BRAKES! Brake light will even come on. Have bled them at all 4 corners. I bench bled the MC, but then let it sit on the bench over night with the passages plugged (nothing leaked out). Could it be that I have to bench bleed it again? I am having a hard time figuring this out...never had this issue before!

You probably have air in the brakes someplace if the idiot light for the brakes is still coming on. Did you bleed the brakes on the 4 corners with the engine on or off? Bleeding is much more effective with the engine on. Second, Have you checked the adjusters on the rear brakes? Some of the Chi-com made bleeder screws on both the calipers and the rear wheel cylinders have a machining issue that prevents a good seat so that the bleeder will suck air when the pedal is released. If you bleed the brakes again and still have soft pedal, start pulling bleeder screws and check for imperfections in the tapered seat.

Dave