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My vehicle has a freshly rebuilt 350sbc with a carb.

I have a vacuum line feeding off the carb heading straight to the brake booster and another feeding the vacuum advance on the distributor. These are the only vacuum lines that are hooked up in the vehicle. The lines are good and have no leaks, there are also no hissing noises in the engine bay.

The brake pedal is hard to press at all times, with the engine on or off, or with any attempted actuation of the brake pedal. With the healthy 350 in drive, I can just barely get enough braking power to stop the car from moving at idle (and this is pushing quite hard on the pedal ). I am used to driving race cars with no power brakes equipped, but this is ridiculous.

The brake booster has a one way valve inline between the booster and the vacuum source. I suspected that the brake booster was no longer good and I replaced it with a used brake booster that was bench tested to be "good", the symptoms did not change. I have not replaced any other component upstream or downstream in the braking system other than front brake pads. I know that one piston, out of the four in each caliper, on the left front is stuck, but not dragging. The vehicle does also have a proportioning valve just after the master cylinder.

What should I be doing to determine the fault in this braking system?

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    Welcome to the site. The brake booster vacuum line should be straight off of the manifold, not off of the carb. The vacuum line on the advance should be above the throttle plate on the carb. Ensure both are right.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jun 4, 2017 at 11:07
  • I believe that mine is working the other way around. What is your reasoning that they need to be switched? Do these ports get different amounts of vacuum pressure?
    – Hatter is Mad
    Jun 4, 2017 at 15:13
  • The brake booster needs direct ported vacuum with a strong vacuum source. This leaves a direct feed off of the intake manifold. If you have it off of the carb (depending on what it's hooked to), you may only be getting vacuum when the butterflies are open. For the distributor, you only want vacuum when the butterflies are open. It's the reason why it's called a vacuum advance. You only want it to advance the distributor when you step into the throttle. Most carbs have a port off the side to hook the vacuum line to for this purpose. I really should make this an answer.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jun 4, 2017 at 16:00

1 Answer 1

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Check the one-way valve is fitted in the correct direction. Also check the port on the manifold is not blocked.

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  • I made sure to check the direction of the one way valve when installing the second brake booster. It is indeed in the correct direction. The line feeding the brake booster is also getting vacuum, also the vacuum advance on the distributor is working so I would have to assume that it also has vacuum.
    – Hatter is Mad
    Jun 4, 2017 at 15:14
  • How did you "bench test" the unit? Is the breather on the other side of the diaphragm clear?
    – Solar Mike
    Jun 4, 2017 at 15:32
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    I just checked the one way valve for a fourth time and sure enough my dyslexic ass had it in backwards. When I get a chance I will go jump the car and make sure that solved the problem. The brake pedal already feels more like it should when it is functioning properly.
    – Hatter is Mad
    Jun 4, 2017 at 17:20
  • I put a fuel filter in backwards and then had to spend 3 hours cleaning the carburettors out, so we've all been there - glad you found it though.
    – Solar Mike
    Jun 4, 2017 at 17:38

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