# '65 GTO Power Brake Diagnosis?



## Roqetman (Aug 31, 2015)

Hello Brake Experts. My '65 GTO power brakes were converted to front disc brakes by the previous owner with stock drums on the rear. When I bought the car the brake pedal was hard and had little feel like you would expect with power brakes but it did stop ok. I adjusted (shortened) the push rod from the brake pedal to the booster trying the lower the brake pedal and hopefully gain more brake feel. Now the brakes feel like there is no boost at all, the pedal is hard and the car does not stop well at all. The booster is getting plenty of vacuum directly from the intake manifold. It has a mild street performance cam. I do not know what brand conversion kit was used as there is no name on the booster or calipers. The discs are drilled and slotted and the master cylinder is a dual reservoir type. There are no leaks and the reservoirs are full of fluid. How's a good way to test the booster to make sure it is working ok? Any advice is greatly appreciated. Thanks!


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## Lemans guy (Oct 14, 2014)

Roque, not an expert on anything,..except chilling out!.... but the standard brake booster Vacumn test is to with car off pump the pedal 3 or 4 times to release any residual Vacumn, it usually will hold vac for 24 hrs or so.

Then put your foot on the brake pedal and start the car, you should feel the pedal drop as Vacumn enters the chamber. If it does not move, then you can walk it back....

What vac does engine pull at brake check valve? The one way check valve on the booster allows air to escape the chamber only when Vacumn is applied...when Vacumn is off that check valve should be closed...

If you think that is OK then check the hose engine vac etc.....

But you must have a good solid brake bleed with no air in the brake lines, yes you can do it by pumping the pedal, we all have but a good brake bleed machine can really make a difference.

So if it is drivable to a garage ask them to put it on their automatic brake blended. They can do it quick and that really gets the bleed right.


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## Roqetman (Aug 31, 2015)

I checked the engine vacuum and I'm getting 12 inches at idle @ 650 rpm. Brake Check valve is ok. With the engine off I pumped the brake pedal 3-4 times, started the engine with my foot on the brake, the pedal does not go down. It only moves about 1 1/4" whether the engine is on or off... not good. I've tried to adjust the length of the push rod but again no improvement. Is this the way it would behave if the booster was bad? Thanks!


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## cij911 (Oct 25, 2017)

Roqetman said:


> I checked the engine vacuum and I'm getting 12 inches at idle @ 650 rpm. Brake Check valve is ok. With the engine off I pumped the brake pedal 3-4 times, started the engine with my foot on the brake, the pedal does not go down. It only moves about 1 1/4" whether the engine is on or off... not good. I've tried to adjust the length of the push rod but again no improvement. Is this the way it would behave if the booster was bad? Thanks!


Here's a link that may be helpful: http://ssbrakes.com/attachment/85206-Vacuum Brake Booster Testing and Diagnosis.pdf

12" of vacuum is not enough to properly run the booster. Generally you want at least 15", with most manufacturers stating minimum of 18". You could add in an electric vacuum (which is what most hot rodders do) to provide plenty of vacuum.

You can buy kits from all the big distributors, ranging in price from ~$130 - 300....

Make sure the check valve on the booster is not leaking....I chased a ghost for a while on my 56' Bel Air only to finally test the valve and determine it was defective...$2 part....

Good luck

Chris


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