Idle speed/smoothness change with brake application

**La Voce** · #1 (**permalink**) 03-15-2008, 05:18 PM

It really is necessary to get out and drive to learn your recent restoration's nuances. I notice that when I depress the brake pedal (Weber carb conversion) the idle speed drops down 200-300 rpm and becomes lumpy, and when I release the brake pedal, she pulls back up and smooths back out. Dang if I didn't realize it has probably been doing this all along while I chased "carburetor problems". I'm thinking the one-way check valve off the manifold to the brake pedal booster needs replaced?

**papajam** · #2 (**permalink**) 03-15-2008, 05:57 PM

I'd think that the check valve is the culprit too. Try plugging or squeezing off the vacuum hose to the booster. If the idle now stays the same with the brakes applied, 99% it's the check valve. The 1% is a bad booster but I'd think that you'd be able hear a hissing sound that pulses with engine speed.

**La Voce** · #3 (**permalink**) 03-17-2008, 05:55 AM

Thanks, unplugged the hose at the booster end (it's sucking as it should), and confirmed no change in idle speed when the brake pedal is depressed. I drove the car with the vacuum line disconnected and plugged and oddly, I like the manual brakes better than the boosted. Yes, it takes a bit more pedal pressure, but it's not too bad, and the feel is significantly better. Anyway, looks like another $70 leaving my wallet for a new valve. Before I do that though, is there a way to be certain it is the valve and not the booster? There's no hissing or moaning noise when I step on the brakes when the line is connected, just the idle speed and smoothness change, which by the way, exhibits the same characteristics regardless of engine idle rpm- I increased it up to 2,500 rpm and with the hose reconnected, idle speed still falls down to barely running.

**papajam** · #4 (**permalink**) 03-17-2008, 08:35 AM

Quote:

Originally Posted by La Voce

...is there a way to be certain it is the valve and not the booster?