Hissing sound under dashboard

[Harry]

Hi. This is for a 03 expy 4.6 this week I started hearing a hissing sound coming from under the dashboard on the driver side. It'll be a quick hissing as soon after I turn the truck on. It'll only happen when I turn the car on after a few hours not being used.

 

[Hamfisted]

Next time you hear the hissing step on the brake pedal and see if the sound changes or stops. Could be a leaking brake booster chamber, or failed check valve on the booster where the vacuum hose from the motor attaches.






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[Harry]

I'll try that. Incase it is the booster will I be able to fix it myself? Or I would I aquire a special tool

 

[Hamfisted]

No real special tools. Just sockets, wrenches, work light for under dash work visibility, etc .... You would need to unbolt the booster from the firewall, and that's accessed under the dash.





.

 

[Motorcity muscle]

If the booster checks out, check the under dash vacuum lines for HVAC controller.

 

[Harry]

Next time you hear the hissing step on the brake pedal and see if the sound changes or stops. Could be a leaking brake booster chamber, or failed check valve on the booster where the vacuum hose from the motor attaches.






.

Tried this method. It kept hissing till I let the brake pedal go.

 

[Harry]

If the booster checks out, check the under dash vacuum lines for HVAC controller.

Will do thank you

 

[Motorcity muscle]

You are welcome, let us know how it works out.