Front will not air up

[travismcclellan73878]

Hi all, I recently purchased a 83 FC-35 and the only issue I've had thus far is the front suspension will not air up. The rear airs up fine but the air pressure will then only build to about 70psi. I've been reading the posts about this issue and I believe it is a dump valve or HCV issue. Any methods on narrowing it down to either? Another peculiar thing is that I can hear what sounds like air leaking from the overhead compartment above the captains seat. Of all of the diagrams I've viewed nothing shows any lines going up there. Could this possibly be a leaking air line to the air horn? Could that be the catalyst to the front suspension not airing up? I'm just hoping it's not an air bag... Any help would be much appreciated. Thanks.

[Frank W.]

Hello and welcome aboard!

Some thoughts/suggestions:

Air leaks in either the brake system or the accessory systems (seat adjust, wheel tilt, suspension, step system) can cause issues...these busses have rather complex air systems compared to other RV's.

Let's check brakes first:

1: Block two wheels well to prevent rolling. Do these tests on level ground.

2: With the CAT running and parking brake set (out) and ideally at a high idle of about 1100 RPM the front and rear air circuits should air up to about 125#, then shortly afterward you should hear a "psst" as the air regulator releases.
If you then turn off the engine...pressure should hold in the brake system (typically several hours at least). Any leak under these conditions would point to supply lines prior to brake pedal.

3: With air at 125#...remove parking brake (tires blocked) and depress brake pedal...air pressure should drop just a bit (like <10), and you should hear no leak if pedal held steady. An audible leak indicates a leaking air line after the pedal...often the rubber hoses leading from the chassis to the brake chamber.

IF you are not able to build pressure above 75#...I'd bet you may have a leaking air bag or air bag line. Cycle the dump/drive air suspension switch a dozen times or so at about a 5 second rate with the system charged as much as it will with air. Then see if you can build pressure set to drive, if no, try dump...see if that helps or allows air pressure to build.

A cautionary note: your parking brake will automatically engage if air pressure drops below about 60-65#. That's also why you can't release the brake until you have that amount of pressure.

As to air horn...suggest cycling it too...many cycles. Get valve/seals moving. These systems sit for ages...use is good.

Let us know results...also check Youtube for videos about "pre trip air brake inspection"...good info there!!!

[travismcclellan73878]

Thanks for the info Frank! Will do as soon as I get a chance to go back out to where it's located. Want to have troubleshooting procedures ready to go.

One more question,

I assume that the front bags are a connected circuit with no check valve, correct? So if one bag or side is leaking it would manifest in both?

[gcyeaw]

The easiest way to find air leaks is to use an external (shop) compressor to air the system up. That way you are not deafened by the CAT. You can remove the drain valve from the wet tank and install an air fitting for this. Later on you should add a dedicated fitting in the system for future issues.

Front bags are one air system, the rear is two. Right/Left leveling is done by the rear bags via two leveling valves..

The compressor feeds the wet tank which in turn feeds the front and rear brake tanks. When the wet tank pressure rise above 65 or 70 the pressure protection valves open and allow air to the rest of the system. That is about where you stop building pressure, so I would guess the main leak is in the accessory system. The leak in the upper front area above your head is pretty likely the air horn system.

[travismcclellan73878]

Appreciate the input. New to these Birds. Trying to familiarize myself with the these systems. I'm really trying to just drive it a little closer to home off of my friend's property so I can properly perform the maintenance required. It's about an hour and half away from anything resembling civilization. The trek back and forth for supplies would be an all day affair. You wouldn't happen to know the size of the drain valve threads on the wet tank off hand would you? If I could take proper size pipe nipple with my compressor it would be nice. 1/4"? 1/2"? NPT?

I'm hoping its just the air horn. Vise grip fix.

[LDM680]

I had something like this happen when checking into a campground. I had shut the engine off, and when I returned after registration, the suspension was down and it would not air up. The air pressure would not get above approx 70 psi on both dash air gauges, the brakes worked, and there was no air pressure port in the storage compartment.

At the campsite, my pancake air compressor would not get the air tanks above 70 psi when previously it had no problem to get up to 120 psi. There was no obvious leaks at the usual suspects, step control , height control valves.

Road service arrived and when he hooked up his large compressor to the coach, air pressure started to go above 70 psi, heard hiss in compartment above driver's head. Tapped the air horn, which stopped the air hissing and pressure continued to climb to 125 psi. on both the dash gauges and the air connector in the luggage compartment. Coach would then air up.

Disassembled the air solenoid and saw a depression on one of the rubber seals that looked like there may have been a speck of dirt that caused the original leak.

So what caused my problem to arise? My dog was sitting in the driver's seat while I was signing in at a campground. He got excited, and briefly tapped the horn button just enough to cause it to make a brief bleet.

Because there was not a big blast of air, a spec of dirt got trapped in the solenoid, which allowed all the secondary air (steps, suspension control, etc) to escape and the pressure protection valve kept the 70 psi in the air tanks.

I am not sure if your condition is similar to mine as you have one axle that airs up, but I there seemed to be some similarities to my situation.

By the way, I found the air horn supply on my coach is the 3/8" plastic line that comes off the mainifold on the underside of the coach just behind the driver's front side bumper.

Good luck in your repair.

[travismcclellan73878]

Wow, haha. I'm hoping it's something as simple. Will be back with updates.

As a side note, being a new owner and recently discovering this forum I'm quite impressed. It has been incredibly helpful and inspires confidence haha. Real stand up bunch of fellows here. Many thanks again and again.

[Rick]

There have been several cases of the winterizing purge valve was being left open. This valve will prevent your system from airing up. Location is on the side of the mattress frame. check it out

[gcyeaw]

Originally Posted by travismcclellan73878

Appreciate the input. New to these Birds. Trying to familiarize myself with the these systems. I'm really trying to just drive it a little closer to home off of my friend's property so I can properly perform the maintenance required. It's about an hour and half away from anything resembling civilization. The trek back and forth for supplies would be an all day affair. You wouldn't happen to know the size of the drain valve threads on the wet tank off hand would you? If I could take proper size pipe nipple with my compressor it would be nice. 1/4"? 1/2"? NPT?

I'm hoping its just the air horn. Vise grip fix.

Should be 1/4 NPT

[Rick Davis]

In addition to other air leaks it could very well also be the front leveling valve. You need to carry one as a spare anyway, so I suggest you pick one up.