As part of my resto, I used a power disc brake conversion kit on the front of my 68. The car drives and stops fine, but runs at high RPMs at highway speed due to 3.73:1 gears. When I slow down on an off ramp or at speeds higher than 20-30 mph, the car stalls as I come to a stop. It restarts, but stalls almost immediately unless I keep giving it gas. The booster has a check valve in it and I have a pretty radical cam in the engine. Do I need a vacuum tank or something? I think my brake booster is siphoning off all my engine vacuum.

Any ideas would be appreciated. It is hard to keep reving the engine at a stop light and keep your foot on the brake and clutch at the same time.

Craig

Did the stalling problem start with the booster install or were there other changes made at the same time?

One way to test if the booster is a problem is to disconnect and plug the vacuum hose to the booster and see how it runs. You will lose power assist so be careful. If disconnecting the hose cures the problem, it's the booster. If there is no change, the booster is not at fault.

The stalling started after the booster was installed and I did perform the booster isolation check you suggested ( I should have included that in my post). I'm pretty sure it is the booster. There are no vacuum leaks. The car is hard to stop without power brakes, so I've only been able to stop it from about 30 mph to test it without the booster. This is the marginal area on the stalling, so the fact it does not stall when I stop without the booster connected, leaves me wondering. If I downshift and stop gradually with the booster connected; no stalling. If I use only the brakes, it stalls. My Z-bar is adjusted correctly, so the clutch not fully disengaging is not an issue. The conversion company tech line had no suggestions other than to check the float in my carb.

sounds like you may need a vacumn tank if you got low vacumn to begin with. or the booster is bad. check vacumn so how much you got to begin with.

Thanks for the advice everyone. I'll have to check vacuum at idle. It's been awhile, but I think it was around 10-12 in. Hg. due to the amount of valve overlap. I had manual brakes when I used to drag race it, so I wasn't as concerned about the driving comfort.

I agree with checking to see what your actual vacuum is.

A vacuum tank will improve booster performance with a radical cam, but will have little if any effect on how the engine runs.

With a good booster, stepping on the brake shouldn't affect the engine at all. The engine only needs to replenish the vacuum when you release the brake pedal.

Try stepping on the brake hard with the engine idling. You should hear a short whoosh of air and the engine should stay running the same. When you release the brake, the engine should change speed momentarily then settle right back down.

If the whoosh of air is continuous and/or the engine stumbles while holding the brake on, you have a bad booster.

Do you have a Holly carb on this car? I had a similar problem with my original Z back in 69 with the stock brake system. It was the front float bowl in the Carb. Any time I had to brake hard the car would conk out. The front float bowls had to have a stiffer spring installed. Just a thought.

Thanks Stewie. The booster checks out fine. I'll need to try red67camaro's suggestion about the float spring. I didn't think the tech line at the brake conversion supplier was on the right path since the car ran excellent with manual brakes. However, the power brakes must allow me to slow down a tad faster and may be causing the float to sink. I have a Holley 650 CFM double pumper installed.

All: Thanks for the help. It is much appreciated.