[MXTex]

The power assist feature of my newly installed brake system is not working. The system is comprised of four wheel discs (C5 up front, LS1 out back). All new lines. New 9" booster, proportioning valve and dual MC. The car is a 1965 GTO. The engine is a 428 with Lunati Voodoo 703 camshaft. It makes 11.5" of vacuum at idle. I realize this is not enough for proper operation of the brake booster. But my issue is there is no power assist at all. I can rev the motor up sufficient to create 18" of vacuum but still no power assist. None at all. The car will stop but takes GREAT effort. Clearly a vacuum pump and/or vacuum reservoir are in my future; but I'm not convinced that my booster is good. Let me explain. In troubleshooting the booster I reached a point where I needed to check to see if it would hold a vacuum. First I attached my mityvac hand help vacuum pump to it. And no matter how much I pumped it, it wouldn't even start to pull a vacuum. Then I simply used my lungs. I pulled on that hose as hard and long as I could and received no resistance at all. Then I hooked up a small electric vacuum pump I have to it and let it run. Still no vacuum whatsoever. And finally, with the engine running and vacuum hose attached to the booster, I removed the vacuum hose from the booster and noticed that the idle didn't change. Then, after placing my finger over the end of the open vacuum hose sealing off the leak, the idle quality changed significantly. So with this information, I was convinced that I had a bad brake booster so sourced another one. I received the new booster yesterday and tested it on the bench before installing. The test results for this new unit were identical to the other. It won't even start to hold a vacuum. Not with the mityvac, my lungs, vacuum pump, or engine vacuum. Nothing. So now I am at a quandary. What are the odds that two brake boosters in a row could be bad in this way? Is my testing rationale? Should the boosters hold vacuum? Do I need to try a higher quality booster? Admittedly, these were the cheapest I could find. Thank you.

[TedRamAirII]

If there is a leak, which there is, where is air ENTERING the booster? where the pushrod goes in? (brake pedal rod) Where the Master is bolted on? You can spray carb cleaner and see where its ingesting air. Put your foot on the brake pedal, start the car, no pedal movement? Booster isnt working. You should feel it "fall away" under your foot. Hard to believe you have 2 bad?

[MXTex]

Thanks Ted. I've tried the pedal test and it does not further depress when the engine is started. And yes, I'm struggling with 2 boosters in a row being bad, Makes me wonder if my vacuum testing is legit. But I can't think of any other way to test for vacuum hold.

[LATECH]

Sounds very likely to me 2 booster are junk. Your testing sounds solid.

[mgarblik]

In the old days we used to teach rebuilding of vacuum boosters. The final test to determine a grade was to "charge" the booster with vacuum from a running engine with a long vacuum hose. The students would push the rod end to "feel" the power assist. Then they would be charged again and put on the shelf with their name on it. The following Monday, each student would push on the rod and feel 1 to 2 power assisted pushes. That confirmed the booster held vacuum. Your testing method is valid. 2 bad boosters. I assume you checked the one way check valves and grommet?

[Brian Baker]

Your testing is thorough, and legit'. Unfortunately you've gotten two bad boosters from your parts supplier. Things aren't what they used to be and parts rebuilders are putting out more and more $h!t all of the time. Two years ago I went through the brakes on my Can Am and had a brand new front caliper from Napa that would not pull brake fluid through the bleeder screw, even with 25" of vacuum on it. In 35 years of playing with cars, that is the first time I've ever gotten a defective caliper from a parts supplier.

[dataway]

Are brake boosters something that can be rebuilt in a well equipped shop? Are there kits available?

[MXTex]

Thanks to everyone for the feedback. It's good to know that my testing is legit. And it's disappointing to know that such low quality parts are being sold to us hobbyist (especially brake components!). So the question that beckons is how do I get a 'for sure' good booster? Should I buy a rebuild kit and have a go at that? Or shall I send it to a rebuilder? Or is there a brand and/or supplier that I can count on to simply send me a good unit. Thanks again for the help.

["QUICK-SILVER"]

Thinking the dual diaphram on the silver car has to have the master cylinder bolted on for the booster to hold air. Got a rubber washer that goes in the hole before the master cylinder slides in. Not positive just thinking....

Since the ones you have don't hold vacuum at all... I'd try applying light air pressure and squirting soapy water on it to look for leaks.

Clay

[MXTex]

Update: The 3rd brake booster worked! What a relief. While I felt pretty confident in my troubleshooting, in the back of my mind I was still thinking; 'what are the odds?' Anyway, it worked. For those that are interested, the first brake booster came with the car. It was still in the box when I bought the car and was part of a disc conversion kit. Also included in this kit was a master cylinder (still in the box), which leaked from the rear seal after installation. Fortunately, I was able to fix the MC as the rear seal was simply twisted. The second booster was identical to the first and the least expensive one I could find from Summit. Summit accepted it's return with no hassles. The 3rd booster was about 50% more money that the first two. As I unpackaged it the first thing I noticed was a sticker indicating it had passed quality control. In summary: one bad master cylinder and two bad brake boosters. All brand new right out of the box. In this case, the price of the item was indicative of quality. I won't make that mistake again. But what a shame that QC issues such as this are so prevalent for of all things; brake parts. Onward to the next phase of this project. Disc brake conversion complete.

[64speed]

I don’t know how much vacuum mines gonna make but it ain’t gonna be great. At what point does hydro boost become necessary

[67gtospud]

Originally Posted by 64speed

I don’t know how much vacuum mines gonna make but it ain’t gonna be great. At what point does hydro boost become necessary

If you’re anticipating low vacuum, you can go the hydroboost route (which I did and absolutely love) or the vacuum pump route. A couple of the guys I cruise with put vacuum pumps on their cars and are happy with the results.

[64speed]

Do not want the noise or hassle of a vacuum pump so it’s hydro boost for me if these don’t work

[ZeGermanHam]

My brother installed a vacuum pump on his GTO, and while it works, I'm generally not a fan and would rather go with hydroboost.