Question about 1989 GMC C1500

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Vacuum line leak

1989 gmc suburban 2500 7.4 l 454 3/4 ton 2 w drive

There is a switch that is activated by the brake pedal and there is a vacuum line attached to it. When the brake pedal is pushed the idle increases and there is an audible sound of an air leak. At times the brake pedal is mushy and then firm. I believe that it is mushy during the leak. At times the vacuum switch will seal and the air leak will go away.
What is the name of the vacuum switch?
What is this switch for?

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Sounds like cruise control cutout switch but dont give me bad rap if i didnt have enough info.

Posted on May 20, 2009

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Hi Monkeypaint!
I assume you have power brakes and I gather that it idles properly at times, but dies when the brakes are applied. This sound like a leaking power brake assist vacuum line or diaphram. You can verify this by locating the big vacuum hose connecting the intake manifold to the assist diaphram. This will be about an inch in diameter connected to the round item behind the brake master cylinder next to the firewall.
  1. Disconnect the line at the intake manifold end, and plug the vacuum port on the manifold with a tapered rubber stopper or cap.
  2. Being aware that you now do NOT have power assist to the brakes (requiring CONSIDERABLY more pedal pressure to stop!!!), see if the problem goes away. If it does, you have narrowed it down to the assist diaphram or the hose. If it still persists, it is an engine tuning/control issue, not related to brakes.
  3. Do determine whether it is the vacuum hose or the cannister, remove the previously installed cap/plug, reinstall the hose to the intake manifold, disconnect it from the diaphram and plug/cap the hose at that end. Repeat step three... if the problem returns, the hose is leaking and should be replaced... If the problem still goes away, the Power Brake Assist Diaphram Cannister is leaking excessively and should be replaced.
  • ***ALSO*** Any other vacuum lines leaking can lower the manifold vacuum to a threshold level where the load of power brake assist actuation crosses the line for adequate vacuum to idle*** So check all vacuum lines underhood, looking for a pronounced hissing at vacuum leaks. A manifold vacuum gauge is a relatively inexpensive diagnostic tool that could also be useful.
Well, hope this helps, and good luck! Don't forget to rate this answer!
Thanks!
-WB

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