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The brake servo or booster is divided into two chambers separated by a flexible diaphragm. Briefly there is vacuum on one side and pressing the brake pedal allows air to enter the other side and this is how the brake pedal effort is boosted.
The air enters via a filter through which the pushrod connected to the brake pedal passes. When the brake is applied the hiss of air entering the chamber will be audible; more on some models than others.
When the forces are balanced; when the brake is sufficiently applied no more air will be required and the hiss will cease. When the brakes are at rest; not applied there should be no hissing.
If there is it indicates a problem with the brake booster, perhaps dirt in the valve gear. Although repairs are possible they aren't very practical to carry out. A repair shop would probably charge more for fitting an overhaul kit than for replacing the unit.
Hard brake pedal when applied with the engine running. A quick test for the vacuum assist booster is to shut off engine, pump brake pedal 3 or 4 times and hold on the last pump, start engine and see if pedal drops a bit....if you detect a problem with booster you should check for adequate engine vacuum supply to booster when idling . You can sometime hear a hissing or air leaking when applying brake pedal with engine running if booster is defective
hissing sound is a vacuum leak somewhere on the power assist part of the system as in a bad booster..try this test..with the car off..push the brake pedal down a few times and release all the vacumm and see how far the pedal goes down..after a few pumps with your foot it should become hard and drop just a bit..once it feels"solid' or hard..hold the pedal down with moderate pressure.. and start the car back up..the pedal should drop a bit further but less than halfway.. if it drops further it needs attention..good luck especially if it goes almost to the floor..
yes but up to a point
the brake pedal should move about 1" to take up the pedal free play and then be applied smoothly as you press further
the harder you press the more solid stopping action you will get
that way you can feel the brakes working so that you can adjust the braking action for stopping
there should be no hissing noise that occurs when the brakes are being applied
you say hydro booster which is hydraulic booster but a hissing noise will come from an air leak in a vacuum brake booster which indicates that it needs repalcing
Your bake booster has failed causing a massive vaccume leak when you apply the brakes. That in turn gives more air to the system so the computer tells it to add more fuel thus causing the rpms to go up. Replace it %99.5 chance that will fix it.
There are 2 things that can use vacuum near the brake pedal. If you have cruise control, there is a safety switch on the pedal to shut off the curise when you apply the brake. This switch may be leaking.
The other part is the brake booster, which is actually in the engine compartment but bolts to the firewall, and your brake pedal mechanism pushes into the booster. You will need to get your head under the dash with the engine running and press on the pedal to locate the leak--on the switch or on the wall.
jcakey: My name is Roger and I will answer your question for you. From what you describe the problem sounds like a bad vacum brake booster. A simple way to check is to do the following. With the engine off. Press on the brake pedal 4 or 5 times. This will deplete all vacum in the booster. Next start the engine while holding your foot on the brake pedal. The pedal should drop a couple of inches. If the pedal does not drop. The problem is the brake booster. When the booster is bad you will hear a hissing sound inside of the truck when you apply the brakes..
If the master cylinder is faulty you will wave mushy brake to no brakes.Your ABS and brake failure lights will be on. Should you need further help please just ask. I will be glad to help you. Please rate this answer as this is how we know your answer was useful.Thank You for using Fix Ya. Roger
Sounds like a vacume leak from power break booster, when you loose vacume it will stall out the engine if enought vacume is lost. chk. power break booster. and hose from engine leading to booster for vacume leak, booster is under hood, break master cyl. bolts to it.
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