Brakes "hiss" when applied - Why?

[edmck101]

We have a 2003 Allegro on a 20,500# F-53 chassis. When I apply the brakes, there's this hissing sound that I first thought might be coming from front brake linings. But then I noticed that the sound also comes when pressure is placed on the brake pedal itself. In fact, I hear the hissing sound increase when I stopped at a light a press harder on the pedal. Now I'm wondering if this sound is coming from the master cylinder; maybe I need to change the brake fluid or look for a broken line. Meanwhile, the brakes are working fine and the coach only has 19,000 miles of use. Does anyone have any experience with this issue? I'd like to know what I'm dealing with before taking it to my local Ford truck dealer. Thanks.

[Campingman]

If it was a vacuum leak in the booster the brakes would be hard to apply. That is the only thing that would make a hissing noise I would think. You can spray WD-40 around all of your vacuum lines. If the Idle decreases you have found your leak.

[ArmyEOD]

sound like your brake booster is going out. It is on the outside your firewall and is right in front of the pedel. It is the drum item that the master cylinder is mounted to.. Not hard to change. Give me a PM and I can tell you how. Or a good garage should do it for around 250.00-300.00 OTD If it fails you will not have power brakes. But you should have brakes just have to push harder (like in the old days before power brakes). When they go bad you can hear the hissing. The air is escaping pass the rubber seal where the brake pod pushes in.

The last one I changed (on my ford pick-up) was about 120.00 plus core charge. Took me about 1 hour to do.

[WSRG]

If the hissing noise can be heard inside the coach,under the dash or out under the hood, when you apply the brakes, it is most likely the vacuum seal on the power brake booster. This seal is the one around where the brake actuating rod goes into the vacuum booster. This won't effect the operation of the brakes until the seal goes completely bad, and I've never heard of the brakes completely failing because of this.
Sometimes, spraying silicone into the tube surrounding the brake actuating rod will stop, or minimize the hissing, but the hissing sound usually indicates that the seal is failing.

Hope this helps.

Bill

[edmck101]

Thank you all very much. Makes me feel a lot better just having an idea of what to tell the Ford dealer to look for.

[tderonne]

Whoa! (Pun intended?)

No vacuum booster = no vacuum leaks.

You have oil pressure assisted brakes. Specifically, oil pressure from your power steering system. Sadly, the noise you describe is normal. Not everyone's seems to be as noisy as the next one, but they all do it. It's just fluid flowing through it. Folks have complained about the noise, changed boosters, even tried to re-engineer the hoses and/or fittings. There's even a TSB for a certain batch of them. No luck that I have seen.

Ford calls the system "Hydro-Boost", but there are a few other names that folks use interchangeably. A Google search will provide more than you can read.

[LVJ58]

As Tim mentioned Ford uses the Hydro-Max/Hydro-Boost for their power brake assist. It functions on hydraulic pressure supplied by the power steering pump, but also has an electric motor powering a pump on the master cylinder should the engine quit running. If you apply pressure to your brake pedal with engine & ignition off, you will hear the pump run which is normal.

[Campingman]

Quote:

Originally Posted by LVJ58 As Tim mentioned Ford uses the Hydro-Max/Hydro-Boost for their power brake assist. It functions on hydraulic pressure supplied by the power steering pump, but also has an electric motor powering a pump on the master cylinder should the engine quit running. If you apply pressure to your brake pedal with engine & ignition off, you will hear the pump run which is normal.

It always amazes me when I learn something after I thought I knew it all.

[subford]

I do not think a 2003 Allegro on a 20,500# F-53 chassis has a vacuum brake booster.

[edmck101]

Again, thank you all for you input on this subject. Seems I do not have a brake problem after all. As far as the hissing noise is concerned, I can adjust to it and will become concerned only if it gets loud. The helpful responses to my original post have saved me considerable inconvenience (a visit to my local Ford dealer) and money (new parts they would probably have needlessly installed).