Brake Booster Question

When I step on the brake, I hear a hissing sound. The engine then runs rough, since it must be leaning itself out.
I believe that this sound is either from the brake booster itself, or the valve that plugs into it. Does anyone know procedures to test either of these things?

Vacuum leak.... Inspect the hose to the booster for any cracks, (all the way from the manifold to the booster). Inspect the seal around the check valve for any leaks. Insure the check valve is not stuck. Blow through the valve to ensure it is not stuck, plugged or letting air move both ways. Soapy water works well for finding vacuum leaks. Either that or have someone hit the brakes while you investigate/listen under the hood. Myself, I wouldn't suspect the booster off the bat (but you never know). Always check the cheap/easy stuff first. Good luck.

Prop, he says the prob occurs when he steps on the brakes. Any leak in the booster vac hose would show up and cause rough idle all the time and any check valve leaking would cause the prob all the time also. I would guess that his diaphragm has a leak in it and is causing the hiss and rough running when braking.

69 Charger RT/SE said:

Prop, he says the prob occurs when he steps on the brakes. Any leak in the booster vac hose would show up and cause rough idle all the time and any check valve leaking would cause the prob all the time also. I would guess that his diaphragm has a leak in it and is causing the hiss and rough running when braking.

Click to expand...

Good point '69 Charger! Pretty critical info I overlooked there. Thanks for the correction.

Here's a procedure from SSBC that I used to test my booster. Maybe it will help you.



Vacuum Brake Booster Testing and Diagnosis

This procedure will require the use of a hand operated vacuum pump with a vacuum gauge. If you do not own one it can often be rented or borrowed from most “big box” parts stores. (Note: 18”HG is the minimum engine vacuum at idle in gear to effectively operate a vacuum booster

1) Remove vacuum hose from check valve on booster. Place hose from vacuum pump onto
check valve and draw booster to 20” of vacuum.

2) Let booster sit with vacuum applied for 5 minutes. If vacuum does not stay steady at 20” it is
faulty and needs to be replaced. If vacuum does hold steady at 20” proceed to step 3.

3) With 20” of vacuum in booster depress brake pedal once and release it. The booster should
transfer some but not the entire vacuum in reserve. Depending on how hard the pedal is
depressed it is normal to see 5-10” of vacuum depleted from reserve. The most important
thing is to ensure the booster does transfer vacuum but does NOT transfer the entire vacuum
in its reserve. If vacuum remains at 20” OR goes to zero the booster is bad and will need to be
replaced. If vacuum transfer is within the above parameter proceed to step 4.

4) Once again draw booster down to 20” of vacuum. Go inside car and depress brake pedal and
hold down for 30 seconds. You should see the gauge drop slightly and then hold steady.
Vacuum should stay steady as long as you are holding the pedal down. If vacuum drops while
pedal is being held down the booster is faulty and will need to be replaced.

Thanks guys, I will test this out soon.

Not sure if it matters, but I also hear the vacuum leak upon acceleration as well.

I know I'm reviving a 4 year old thread; came across it doing research on a current issue I'm having and this one looks like it never got completely resolved, so here goes...
During the resto on the Coronet I swapped over to the Right Stuff disc brake conversion kit along with the compatible Bendix style booster. It's been 3 years since the install but lately there's been the hissing coming from behind the brake pedal. Per post #5 above I hooked up a vacuum pump to the check valve (engine not running) and wasn't able to draw any vacuum (I verified the pump off the car - vacuum pump and hose held vacuum). All signs point to a damaged diaphragm. It seems odd because it's only 3 years old and I've only racked up about 3000 miles.
Any other troubleshooting to do before pulling the booster? Thx.