KJL

I noticed that when I perform a vacuum check on the power brake vacuum booster, the vacuum holds until I push the brake pedal in. As I depress the pedal, all vacuum bleeds off and the booster is is nothing more than a vacuum leak. I used an electric vacuum pump to perform this test. Is it supposed to function this way?

DUB

YES...that is how it works....which is why you can hear a slight 'whoosh' of air sound when you press on your brake pedal when you are listening at the brake pedal area.

DUB

Scott Liggett

Before you go spending money throwing parts at your Corvette in hopes of fixing your problem, first post up what cam you are now running with the cam specs. Low vacuum reading can be caused by more than just a bigger cam. I have run cams with 242/248 duration @.050 with an 850 DP and still have enough vacuum for brakes.

You may just need to do something very simple to get your vacuum reading back up to where your brakes function correctly. Things like lack of ignition timing, or an overly rich carburetor setting can kill vacuum, as well as having the valves adjusted too tight. All can be fixed without spending money on new parts.

Scott Liggett
BluePrint Engines
Product Specialist

KJL

My cam is 257/263 at .05. I run a locked distributor with timing set to 34 degrees. No vacuum leaks and carb is a 950 Holley with mech secondary’s. I tune with a wide band O2 meter. My vacuum is around 10” at best ant that is at a 1200 rpm idle. If I tinker with it I can get vacuum to close to 12 inches but that leans it out a little too much.

Scott Liggett

What size engine is this?
257/263 duration with anything less than 112* lobe separation angle will give it one heck of an idle note, but all that overlap will be tough to make vacuum.
If this is a street car, why are you running the distributor locked out?
10-12 inches of vacuum at idle is borderline for brakes. Do you have any vacuum assist at all? If so, maybe all you need is a vacuum resevoir can.

KJL

It is a 434 small block. Engine dyno at 700 HP and 570 on a dynojet at the rear wheels. The best I can do is about 10 inches. I am going to try some Hawk performance competition brake pads on it to try and improve the “grab”.

gmcman52

some one mentioned vacuum reservoir tank could help. I installed one on my vette and it doesn't help much. once the can is full it works ok and then it doesn't full up quickly. so then you have a spongy brake pedal. Some member's run a vacuum pump and claims it works great but is noisy. I'm looking at installing a leeds vacuum pump which is not as noisy and puts out 25 PSI. check out their web site and review their video.

Scott Liggett

Just be careful to pick the brake pads for street use. Some race versions don't work very well until they get hot. Very scary on the street and would make matters worse. I am sure Hawk's website will have such warnings.

Bullshark

I had the exact same problem with my 69 540ci Dart 750 hp Blue convertible. Installed Wilwood brakes and master cylinder but kept the old booster, which I assumed was still good. I installed a vacuum pump to supplement low vacuum from engine. One push of he pedal and vacuum assist was toast. Had to wait till pump recovered. I installed a new Tuff Stuff dual diaphragm power booster and problem is fixed. No loss of vacuum to the extent that can't recover rapidly. All I can say is the old booster must have had a vacuum leak during assist.

Bullshark.

Gunfighter13

"It is a 434 small block. Engine dyno at 700 HP and 570 on a dynojet at the rear wheels. The best I can do is about 10 inches. I am going to try some Hawk performance competition brake pads on it to try and improve the “grab”."

I think your at the point where you need to convert to manual brakes or convert to Hydro boost assist.