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xsvector, Auto Mechanic
Category: Dodge
Satisfied Customers: 15040
Experience:  asetech
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I have a ssing noise coming from under the dash of my 2008

Customer Question

I have a hissing noise coming from under the dash of my 2008 dodge truck. I am not sure where it is coming from. It goes away if I press the brake pedal or turn the steering wheel. I am not getting any lights and I cant notice any difference in the ride or performance. What is causing the hiss?
Submitted: 7 months ago.
Category: Dodge
Expert:  xsvector replied 7 months ago.

sounds like the brake booster has a vacuum leak.. when the brake booster leaks it requires replacement

Customer: replied 7 months ago.

How does that effect the steering? the hissing goes away when I turn the wheel.

Expert:  xsvector replied 7 months ago.


The hydraulic booster uses hydraulic pressure from the power steering pump. Before diagnosing a booster problem, first verify the power steering pump is operating properly. Perform the following checks.

  • Check the power steering fluid level.
  • Check the brake fluid level.
  • Check all power steering hoses and lines for leaks and restrictions.
  • Check power steering pump pressure.


The hydraulic booster unit will produce certain characteristic booster noises. The noises may occur when the brake pedal is used in a manner not associated with normal braking or driving habits.


A hissing noise may be noticed when above normal brake pedal pressure is applied, 40 lbs. or above. The noise will be more noticeable if the vehicle is not moving. The noise will increase with the brake pedal pressure and an increase of system operating temperature.


A clunk-chatter-clicking may be noticed when the brake pedal is released quickly, after above normal brake pedal pressure is applied 50-100 lbs.


With the engine off depress the brake pedal several times to discharge the accumulator. Then depress the brake pedal using 40 lbs. of force and start the engine. The brake pedal should fall and then push back against your foot. This indicates the booster is operating properly.


  1. Start the engine, apply the brakes and turn the steering wheel from lock to lock. This will ensure the accumulator is charged. Turn off the engine and let the vehicle sit for one hour. After one hour there should be at least two power assisted brake application with the engine off. If the system does not retain a charge the booster must be replaced.
  2. With the engine off depress the brake pedal several times to discharge the accumulator. Grasp the accumulator and see if it wobbles or turns. If it does the accumulator has lost a gas charge and the booster must be replaced.

SEAL leakage
If the booster leaks from any of the seals the booster assembly must be replaced.

  • INPUT ROD SEAL (3): Fluid leakage from rear end of the booster.
  • PISTON SEAL (6): Fluid leakage from vent at front of booster.
  • HOUSING SEAL (4): Fluid leakage between housing and housing cover.
  • SPOOL VALVE SEAL (7): Fluid leakage near spool plug.
  • RETURN PORT FITTING SEAL (8): Fluid leakage from port fitting
Expert:  xsvector replied 7 months ago.

hope that s a hydro boost set up

Customer: replied 6 months ago.

Ok, I have checked everything. There are no leaks, no jerking when steering or change in the break pedal. The pedal still has a boost for a couple of presses after the truck has been sitting. The hiss goes away as soon as you apply pressure to the pedal or to the steering.

Expert:  xsvector replied 6 months ago.

its from the hydroboost set up..either jump the gun and replace or wait until it fails

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