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Brake Booster-Master Cylinder-Loose hose??

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Old 09 Mar 2017, 12:27 pm
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Question Brake Booster-Master Cylinder-Loose hose??

Here's the facts:

2005 PT Base model. 2.4L engine, 144K

I've noticed for the last little bit, that while driving along, I can hear a hissing sound coming from seemingly underneath the dash or right behind it more to the left side. The hissing sound will stop with even the slightest pressure applied to the brake pedal.

I've seen no difference in braking ability, no hard pedal. Engine seems to run fine.

I will note that within the last month or so, I did get a O2 sensor malfunction that went away on its own.

I have had the brake booster replaced within the last year, so I'm having a hard time believing it's that.

What do you guys think? I'm inclined to think that a vacuum line may have come loose. The Almighty, my father, seems to believe its a bad brake booster. If anyone can offer any insights into what the issue is and what it would take to fix it, would be greatly appreciated.
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Old 09 Mar 2017, 12:38 pm
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Default Re: Brake Booster-Master Cylinder-Loose hose??

First thing to check is the 3/8" diameter vacuum hose that runs from the booster to a connector under it or off to the side of it. I'm not sure which on the N/A engines. Make sure to check EVERY vacuum line in the area. You may just have one that's slightly cracked and leaking and not actually broken.

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Old 09 Mar 2017, 06:38 pm
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Default Re: Brake Booster-Master Cylinder-Loose hose??

Check your fresh air/ recirculation control on the climate control panel.

This is controlled by a small vacuum line running from a canister off the brake booster, to the left side of the heater box under the dash.

When you move control to recirculation position, you should hear the door in the heater box move with authority and stay there.
Vacuum holds the door against spring pressure opposing the vacuum.
When the motor is off, the spring returns the door to fresh air position.

It is possible that the sound comes from the vacuum hose connection to the back of the climate control panel or the actuator on the heater box (behind glove box (IIRC).
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Old 16 May 2017, 01:27 pm
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Default Re: Brake Booster-Master Cylinder-Loose hose??

so it's been awhile, and the problem still continues. I've not had time to do as suggested, but I wanted to offer some more incite into the situation.

I did poke my head up under the dash, and found the brake pedal assembly is connected to an air-filled accordian looking thing that passes through the fire wall. This is where my hissing is coming from. Would this influence the previous posts thoughts?
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Old 16 May 2017, 02:13 pm
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Default Re: Brake Booster-Master Cylinder-Loose hose??

If you know that is where the sound is coming from, it sounds like your booster. You could try pulling the vacuum hose loose from either the booster or the engine, cap it off temporarily as to not loose engine vacuum and see if the noise persists. Can you hear it while the car is sitting? If you have to drive it to hear the sound, I would only do so under a very controlled situation. Brake pedal is hard to push and stop the car without the vacuum assist.
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brake booster, hissing, master cylinder, vacuum leak

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