View Full Version : Air brake question

12-08-2009, 12:57 PM
I haven't written in a while, but then, a new grandson will do that to your schedule/routine.

Anyway, we're preparing for our winter trek out of the cold and I noticed something new on the coach.

I took the coach for it's annual safety inspection and the mechanic pulled all four wheels and brakes are good. Braking is sure, it stops as expected and the pedal is solid. However, when I apply the brakes, I now hear a constant hiss. When I release the brakes, the hissing stops. I don't recall this before.

I just did a little test in the driveway with the wheels chocked. Started the vehicle and allowed the air pressure to reach max (both front and rear gauges read around 125 psi).

I applied the brake and held my foot on it for almost 3 minutes. Constant hiss. At a little over 3 minutes, the needle for the front reached 65 psi and the low air alarm sounded. Rear air was just slightly higher at around 70 psi.

When I removed my foot from the brake pedal, the hissing stopped and the pressure immediately began to build. The alarm stopped almost instantly and in about 45 seconds pressure was at max (verified by the sound of a short release of air) and no hissing until my foot depressed the brake pedal again.

Do I have a problem?

Beaufort NC
12-08-2009, 08:20 PM
Check Rate of Air Pressure Buildup. When
the engine is at operating rpms, the
pressure should build from 85 to 100 psi within
45 seconds in dual air systems. (If the vehicle
has larger than minimum air tanks, the
buildup time can be longer and still be safe.
Check the manufacturer's specifications.)
In single air systems (pre-1975), typical
requirements are pressure buildup from 50 to
90 psi within 3 minutes with the engine at an
idle speed of 600-900 rpms.
If air pressure does not build up fast enough,
your pressure may drop too low during driving,
requiring an emergency stop. Don't drive until
you get the problem fixed.
Test Air Leakage Rate. With a fully-charged
air system (typically 125 psi), turn off the
engine, release the parking brake, and time
the air pressure drop. The loss rate should
be less than two psi in one minute for single
vehicles and less than three psi in one minute
for combination vehicles. Then apply 90 psi
or more with the brake pedal. After the initial
pressure drop, if the air pressure falls more
than three psi in one minute for single vehicles
(more than four psi for combination vehicles),
the air loss rate is too much. Check for air
leaks and fix before driving the vehicle.
Otherwise, you could lose your brakes
while driving.
Section 5 AIR BRAKES
New York State Commercial Driver?s Manual

Here's the link:


12-08-2009, 10:00 PM
My gauges only have 4 psi markings on them 0 - 65 - 100 & 150. As I stated in my original post, when I released my foot from the brake, there was no more air loss and the pressure went from 65 to 125 in about 45 seconds. According to your info, the return rate should be 15 psi in 45 seconds. Since mine recovered about 60 psi in 45 seconds, I assume the result is excellent.

My concern was the loss of air when the brakes are applied. I can't imagine having to fully apply my brakes for more than 3 minutes, with the possible exception of being stuck at a long red light.

When the brakes are released, there is no air loss at all. Additionally, there doesn't seem to be any, or at least any noticeable air loss when brakes are applied partially, as when slowing down.

In the morning I'll try to estimate the rate of air loss per your suggested method.


12-08-2009, 10:50 PM
Heritagerider sounds like you have a leak in your brake valve. they will go bad if condensation or rusty air grunge gets in the valve. have pumped the brake fast and sometimes this will stop the hissing but it is only a short fix . you will have to take your MH to a shop to R&R the foot valve. what chassis do you have? If its a Freightliner or Spartan they can help you right away or most truck shops as the units are mostly standard. if you are handy you could get the part at a truck parts store and do it yourself. hope this gives you some options.

12-09-2009, 07:56 AM

Thanks for the response. I can be pretty handy. Can you tell me how to access the part in question? It is a Freightliner chasis

I can tell the hissing is coming from under the front center of the coach, where I think the tank is located.

12-09-2009, 10:21 AM
my MH is covered with snow and its 20 below celsius so i can't get to it but i think the way in is with the genny out you will be able to see the valve body under the floor. it should have a number of air lines coming out at 90 degree angles one of which will be the air line for brake pressure release which is i think what you hear when you apply the brake. if you can't get to it that way turn your wheels far left or left wheel off if you can and that will be your way in. i wish there was not so much snow around mine i would have a better idea as to how to get to it. i have changed two in my own trucks but not in a MH. its not a big job just mark your lines so they go back the same way. help is available at your freightliner truck shop so ask them for ideas and i'm sure they will be happy to assist you they are good people. PS. good to hear you ride two wheels my wife and i are holdouts my valkrie & her shadow but both my boys ride HD's how is it to pull a trailer full of goodies would like to set up that way myself Jack

12-09-2009, 11:13 AM
Have you checked your drain valves on the resvoirs? Maybe one of your lanyard cables might have gotten struck by debri and the valve might be partially open. They are in front right behind the front axle.
Another thing I would do is to start the engine and let the system charge up. Then I would get somebody to get inside the coach turn the engine of while you crawl under the coach. Have the helper press on the air brake pedal and check for leak while under the coach. It could be that a cylinder line might have come lose and is leaking. Those lines only leak when the pedal is being pressed.

12-09-2009, 12:54 PM

Thanks for the practical checks. I'll try them before taking it to a shop.

12-13-2009, 12:10 AM

Thanks for the practical checks. I'll try them before taking it to a shop.

Let us know what your problem was with the coach. It might help some other Gulfstream owner that might have the same problem.

12-13-2009, 07:42 AM

I'll do that. Just waiting for a decent day weather wise. Been below freezing the past few days and this morning there is a thin coat of ice on the driveway.

Got to get it fixed so we can get to Florida and escape the cold.

I'll keep you posted.

12-13-2009, 03:49 PM
Got to get it fixed so we can get to Florida and escape the cold.

I'll keep you posted.

Come on down! It was 85 degrees here today in central Florida.

12-13-2009, 06:54 PM
Come on down! It was 85 degrees here today in central Florida


Can almost taste it now. Just got to get past Christmas, then we're on our way. Been keeping an eye on the weather and saw it was nice in east central Florida.

12-15-2009, 11:25 PM
Yes Florida can be nice this time of the year. Still rains a little too much for me. I have a son that lives in Miami Shores and when we go down there to visit him we usually stay at Penny Thompsons RV park. It is a real nice RV park right before you enter into the Florida Keys. I believe it is owned by the county. We also stay at 1000trails in Orlando many times. This time of the year I prefer Palm Springs, California over Florida.

12-15-2009, 11:33 PM

I'll do that. Just waiting for a decent day weather wise. Been below freezing the past few days and this morning there is a thin coat of ice on the driveway.

Got to get it fixed so we can get to Florida and escape the cold.

I'll keep you posted.

I notice on your signature that both you and your wife ride Harley's. I road an Electra Glide for many years plus other brands, but I am thinking of going back to Harley Davidson. I am undecided wheather to get a full dresser again or go with the stripped version. Something like a lowrider or night train. I have being asking questions around if I can carry a low rider on a hydraulic ramp in between my coach and my toad which is a Honda Pilot. Would it have any effect on the steering? My coach is the Scenic Cruiser 38' Diesel Pusher.

12-16-2009, 08:17 AM

We'll be doing California next year. Ventura area

If you're torn between a full dresser and a stripped model, I'd look at a Street Glide. Low, clean and still has many of the dresser toys.

As far as the hydraulic lift, we had an Overbilt Lift http://www.overbiltlifts.com/ on our old 29' Hurricane (no slides) on a Ford chassis. Had to do some suspension mods on the Hurricane. The extra weight on the tail made the nose squirrelly.

When we got the 41' GS, I had the lift transferred to it and honestly, I didn't even know the thing was back there. We were pulling a Chrysler Sebring on a tow dolly and then went to a tow bar.

If you get a lift, and are using a tow bar, be sure to get a drop down hitch for the toad. The lift receiver is a bit high and the angle of the tow bar to the bumper can be too steep.

Now, with my bike and her trike and the car, we have to have a trailer. And frankly, I prefer the trailer. Everything arrives clean and dry and extra room for stuff!

12-21-2009, 03:15 PM
Well, dealing with cold and snow and holiday stuff, I finally got someone who knows what they are doing to look at the MH. Turns out the leak is a crack in the airline. Took it to a repair shop today. They think they can fix it tomorrow.

12-23-2009, 12:29 PM
Well, it was a Quick Release Valve???

$77.00 in parts - $115.00 labor

It's done and ready to leave for Florida on Sunday - where, by the way, it is 40 degrees warmer than it is here.


12-25-2009, 11:03 AM
That is great that you got the brakes fixed. I figured it had to be some kind of valve stuck open or a crack on the air line.
I worked on the air brakes on trains for many years, so I figured that air brakes on our coaches are basicly the same thing. Not as complicated, but same system.