[alanmay0]

OK I have a 1967 GTO. Today I noticed that when at a red light with my foot on the brake my idle drops about 100 RPM's. Did some research on the web and did the put your foot on the brake peddle and start the car and if the brake pedal goes down your booster is good which it passed. Also read to disconnect or pinch off the vacuum line running to the booster and then turn on your lights to see if you have some sort of electrical problem causing the alternator to work harder when foot of brake. SOO I tried that, no change in idle. I have been having trouble adjusting my QJET because I would have to turn one of the air/fuel mixture screws almost all the way in to get the car to somewhat smooth out. WELL when I pinched off the vacuum line going to the brake booster my engine idle went way down like if you turn one of the air/fuel mixtures screws all the way into the carburetor. Un-pinch the vacuum hose and the idle goes back up. I am thinking that this indicates a leaking power brake booster, even thought it passes the above mentioned test. It may be a small leak but still a leak. Any thought on this problem.

[6x400gmc]

I've had a leaking booster on an 80 Bird before but it would squeal as the air passed through a crack in the diaphragm. Not sayin yours isn't leaking, but may want to try and listen to it. Also, check the hose, check valve, and check valve grommet as they can get hard over time and leak as well.

Britt

Sent from my SM-N950U using Tapatalk

[PunchT37]

Man, you better stay out of Vegas.

[alanmay0]

Thanks just replaced the hose with correct vacuum hose vs the brake like hose that was on it. Also checked check valve and it was good. Have not checked the grommet.

[STEELCITYFIREBIRD]

Your carb is adjusted to compensate for the vacuum leak!If grommet checks out booster is done. (v likely booster). I've encountered check valves that were damaged, glue joint seperating and caused a vacuum leak to atmosphere.. usually newer replacement junk.

Try this test, if your sure check valve is holding solidly..grommet tight. Run engine rev a few times to build up booster vacuum storage, shut off car, you should have one, or two stabs at the pedal with assist, pedal travel higher and then hard pedal after vacuum stored in booster is used up.No assist engine off, vacuum leak within booster.

My car had high idle when I pressed brake when booster went.Went straight right to the booster.
HTH

[NeighborsComplaint]

I had to replace my booster which was only about 5 years old (Delco Moraine stamp). I had unresponsive mixture screws, low vacuum and soft pedal. The pedal drop test was normal on startup. After changing the booster, the carb and brakes responded normally.

[lust4speed]

My booster lasted 50 years on my '67. I caught mine with my foot on the brake and shutting off the engine. Things should remain constant, but when the engine vacuum was removed from the booster, the brake pedal pushed back hard against my foot. A good booster will just sit there with the brake pedal held constant and engine is shut off. So my booster was also a steady vacuum leak. Anyway, one more test you can do.

[alanmay0]

Thanks same symptoms I am having. I have order a new booster. Will let you know what happens. My booster is only 2 year old.