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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Hi, your basic symptoms will be. Your brake pedal. Will be very hard to push. When the pedal is engaged there will be a large vacuum leak, usually causing the engine to be very ruff or stall. If its bad enuff, youll hear the the air leak when pedal is depressed, hope that helps.
you mean you hear it WHEN you push pedal ? power booster could be going bad ! check allvacuum lines, sound good ? any trouble with brakes ? hard pedal ? tighten throttlebody mounting bolts ! gaskets do go bad, they get hard, dryout, become brittle, vacuum leak !!! replace vacuum lines if cracked, dryed out, etc.,, you can use a 3 to 4'-0 pc. of hose to listen hissing sounds where vacuum occurs to locate the leak .
The older vehicles used vacuum to control the Cruise. Check for a leak in the vacuum system. Using a can of starting fluid, spray the vacuum lines and fittings lightly. If the engine revs up after a spray, it'e most likely the leak.
the booster is the problem, or a hole or leak in vacuum line to booster check the check valve also ! from manifold. heres the symtoms of a bad brake booster. primary symtom is a hard pedal on application, to test booster. pump brakes several times with engine off. to deplete stored vacuum. turn on engine with pushing slightly on brake pedal. you should feel the pedal fade away abit and then become firm. but not hard. if you feel nothing at pedal when engine starts, brake booster is not working! check the vacuum lines first!!
Try unplugging the vacuum line to the brake power booster with the vehicle in park and running. Press the brakes if it feels the same then the vacuum booster is bad and needs to be replaced. Try it with the vehicle running, and then not running. If the booster is boosting(for lack of a better term) there will be a difference when it's running.
You likely have an internal leak in the booster. Unplug the vacuum line from the booster, plug the rubber line with a bolt or something and see if the eliminates he noise. If it does, then your booster is bad.
Run the vehicle at idle and turn off engine. Step on the brakes with the engine off and listen for vacuum leak. If you hear hissing with the brake pedal depressed it's the Brake vacuum booster......In the same area there is a parking brake vacuum release switch which leaks only with the car is in idle. If this is leaking your automatic parking brake will not release your parking brake automatically in drive.
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.
Disconnect the line at the intake manifold end, and plug the vacuum port on the manifold with a tapered rubber stopper or cap.
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.
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!