Not your typical brake question!

[Scoundrel]

Hello,

I have a question regarding brake noises. I have a 2004 Forest River Georgetown and I just recently picked it up from the dealer. It was in for service on the Genset only. As I was on the way home I notices that when I apply pressure to the brake pedal there is a very light hissing sound coming from somewhere under the dash. I only recently purchased the MH so I don't know if this is normal for this particular model. I have no problems stopping, in fact it stops very well so is this a normal sound or does it indicate a problem?

 

[John From Detroit]

If the rig is a gasser then the power assist brakes work on vacuum,  Now,  a brief "HISS" sound when you press the pedal is not uncommon, but if the hiss continues then you have a leak in the booster cylinder and need to replace it -OR-........

You have a bad valve on your cruise control servo and need to replace it

However I'm not sure you would hear that hiss.  I've only had two Cruise controls go bad, neither one hissed when I pressed the brake. (One was a bad valve and the other a bad solenoid housing)

 

[Karl]

Scoundrel,
A crack in the outer shell OR a tear in one of the two internal diaphragms of the booster OR a leaking vacuum line from the manifold OR a bad one-way valve which connects the vacuum line to the booster will cause a hissing noise when the pedal is applied. Have someone place an ear near the booster as you apply the brakes to see if that is where it's coming from. A further test would be to start the engine and let it idle for a minute or two, then shut it off. Apply the brakes slowly but steadily until the pedal is quite firm. Release and repeat. you should get at least one and one-half applications before the pedal stays near the top with moderate pressure. Next, pump the pedal until it is very hard, then, while still applying pedal pressure, start the engine. The pedal should go down a minimum of 1". If either of those tests give incorrect results, it requires further testing by a specialist. 

 

[Karl]

Scoundrel,
There is one more test you can do, but it requires driving the vehicle. Do the previous tests first. If they don't show the problem, I'll give you the last test procedure.

 

[Scoundrel]

Scoundrel,
A crack in the outer shell OR a tear in one of the two internal diaphragms of the booster OR a leaking vacuum line from the manifold OR a bad one-way valve which connects the vacuum line to the booster will cause a hissing noise when the pedal is applied. Have someone place an ear near the booster as you apply the brakes to see if that is where it's coming from. A further test would be to start the engine and let it idle for a minute or two, then shut it off. Apply the brakes slowly but steadily until the pedal is quite firm. Release and repeat. you should get at least one and one-half applications before the pedal stays near the top with moderate pressure. Next, pump the pedal until it is very hard, then, while still applying pedal pressure, start the engine. The pedal should go down a minimum of 1". If either of those tests give incorrect results, it requires further testing by a specialist. 

Karl,

Assuming one or more of these causes exist what kind of repair cost can I expect?

 

[dave95.1]

Before you go chasing that I would make sure you have vacuum boost.  My gasser has hydroboost and a hissing sound is normal with hydroboost.  Take a look at your master cylinder and see if what it mounts to has hydraulic lines from the powersteering pump or a vacuum line from the engine.  I don't know what chassis you have but alot of GM chassis use hydroboost.  Have no idea about what the fords use.

 

[Gary RV_Wizard]

Which chassis is the Georgetown on? Workhorse or Ford?  If Workhorse, it would help to know if it is the P30 or the W20/W22, since they are quite different mechanically. The W20/W22 will have a 5 speed automatic transmission, the P30?p32 is a 4 speed, if that helps.

 

[Scoundrel]

Gary,

ITS A FORD !....

I had an inspection done and they guy never mentioned a problem with the brakes. I put in a call to him to see if he knew about the sound and if it was expected. 

 

[Scoundrel]

Scoundrel,
A crack in the outer shell OR a tear in one of the two internal diaphragms of the booster OR a leaking vacuum line from the manifold OR a bad one-way valve which connects the vacuum line to the booster will cause a hissing noise when the pedal is applied. Have someone place an ear near the booster as you apply the brakes to see if that is where it's coming from. A further test would be to start the engine and let it idle for a minute or two, then shut it off. Apply the brakes slowly but steadily until the pedal is quite firm. Release and repeat. you should get at least one and one-half applications before the pedal stays near the top with moderate pressure. Next, pump the pedal until it is very hard, then, while still applying pedal pressure, start the engine. The pedal should go down a minimum of 1". If either of those tests give incorrect results, it requires further testing by a specialist. 

Karl,

I did as you asked however something else is now part of the mix.....With the engine off as soon as I begin to depress the brake pedal the sound of a small electric motor starts. And the hissing sound has not begun yet. As soon as I turn the ignition Key on without starting the engine the same small electric motor begins. I don't know if that has anything to do with the brakes or not. Any ideas? 

 

[dave95.1]

That sounds just like mine.  Hydroboost with an electric pump for the booster in case the engine dies.

 

[Karl]

Scoundrel,
Can't give you a cost without knowing which unit we're talking about or the time required to replace it. It's not a hard job, but access may be limited. I also can't comment on hydroboost systems, but can provide this link to an excellent article about them, which may or may not apply to your situation:Hydroboost

 

[Gary RV_Wizard]

The motor powers a small pump that provide brake boost when the engine is off. That's so you can safely brake to a stop if the engine dies.

Last Ford chassis motorhome I owned was a 96 and I don't recall a hiss when stepping on the brake pedal. Don't know if it is normal for newer ones or not but dave95.1 says his does the same. 

 

[Karl]

Scoundrel,
My F53 class A Ford chassis manual states the following:

"Normal hydro-boost will produce certain characteristic noises that current technology cannot correct. These noises occur, for the most part, when the brake pedal is manipulated in a manner not associated with the everyday braking habits. The general categories of normal operating noise are hissing noise and clunk, click and chatter noises.

The hydro-boost will emit normal hissing noises when above normal pedal efforts are applied. The hiss sounds are particularly noticeable with the vehicle motionless and will increase in intensity as pedal efforts increase. Loud hissing sounds at or below normal pedal effort (111-156N [25-35 lbs]) warrant investigation and possible correction.

Clunk, chatter or clicking noises will be heard when the brake pedal is quickly released from hard pedal efforts. Due to "quick release for [sic; should be 'from'?] hard application" requirement for the noises, they will most likely be heard with the vehicle motionless or during a fast stop-start condition."

The manual also states that class C's will use the vacuum booster; not the hydro-boost.

Added: There is no mention of an electric pump for 'dead engine' situations. The system uses a pressurized accumulator to provide at least 2 stops without the engine running. What you may be hearing is the fuel pump?

 

[dave95.1]

The hissing noise is the sound of the fluid going thru the booster.  It shouldn't be real loud, but is noticable once you get tuned to it.  On mine, the accumulator has an electric pump that works if you step on the brake with the engine not running.  You're talking in the neighborhood of 1500psi that the power steering system can operate at.

 

[Scoundrel]

The hissing noise is the sound of the fluid going thru the booster.  It shouldn't be real loud, but is noticeable once you get tuned to it.  On mine, the accumulator has an electric pump that works if you step on the brake with the engine not running.  You're talking in the neighborhood of 1500psi that the power steering system can operate at.

I got this confirmed at a fleet repair shop that does all the city buses and heavy trucks. The hissing sound is annoying but normal and there is a electric pump that serves as a back-up in case of engine failure........

Thank you everyone for your input and research.