Air leak for beginners

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Air leak for beginners

June 09, 2021, 01:33:34 pm

Without getting too technical, what is the best way for me to start looking for an air leak on my U295? Before leaving on this trip, the coach would maintain pressure over 100 for probably 2 weeks. Now I go from 120 down to 35 overnight.
Also, I have 4 safety stands (12" trailer hitch receivers from Harbor Freight) but should I really have 8 if I want both front and back up high at the same time?

Re: Air leak for beginners

Reply #1 – June 09, 2021, 01:51:02 pm

On the safety stands, yes, use 8 stands when raising both ends of the coach.

Hunting for leaks. Use your air system diagram to come up with a logical method of going through the whole system. I would suggest starting at the beginning and working down stream. All air pressure originates at the engine air compressor. There are check valves in the system that are supposed to keep the air moving the proper direction. As you work through the system, verify the operation of the check valves. As you work past each (known to be good) check valve, you can eliminate everything up stream from that valve as a source of your leaks.

Some members have had good results with inserting extra "test" 1/4 turn ball valves into the system. By placing a valve at a strategic point, they can isolate that section of the system. Helps in checking the condition in long runs of air line that may not be easily accessible.

Re: Air leak for beginners

Reply #2 – June 09, 2021, 02:25:04 pm

Separate the air into leveling system and everything else. If the bus drops as pressure drops there is a check valve/air bag. air bag control issue.
Often the park button will drop pressure.
Run the engine, check the needles and see if one falls faster than the other and try to determine if the loss is one end (needle) or both.
Keep in mind that loosing air overnight is common. Dropping air pressure is measured by # per minute. 60 #s per 60 minutes is a little high for me but may still pass the DOT test .
I get about 100 #s loss in a week on one side and 50 on the other side.

Re: Air leak for beginners

Reply #3 – June 09, 2021, 04:36:30 pm

Quote from: Chuck & Jeannie – June 09, 2021, 01:51:02 pm

Use your air system diagram to come up with a logical method of going through the whole system. I would suggest starting at the beginning and working down stream. All air pressure originates at the engine air compressor. There are check valves in the system that are supposed to keep the air moving the proper direction. As you work through the system, verify the operation of the check valves.

Where do I find an "air system diagram" for my coach? I don't have one.
And how do I "verify the operation" of the check valves?

Re: Air leak for beginners

Reply #4 – June 09, 2021, 04:41:28 pm

I hope this helps. This is for a 99 U320

Re: Air leak for beginners

Reply #5 – June 09, 2021, 04:51:00 pm

Quote from: Protech Racing – June 09, 2021, 02:25:04 pm

Separate the air into leveling system and everything else. If the bus drops as pressure drops there is a check valve/air bag. air bag control issue.
Often the park button will drop pressure.
Run the engine, check the needles and see if one falls faster than the other and try to determine if the loss is one end (needle) or both.
Keep in mind that loosing air overnight is common. Dropping air pressure is measured by # per minute. 60 #s per 60 minutes is a little high for me but may still pass the DOT test .
I get about 100 #s loss in a week on one side and 50 on the other side.

Pretty sure the coach doesn't drop until the dash gauges drop to 60 or below. I'll test that again today.
I noticed something happen a few days ago that I had never seen before. My wife was driving and I glanced at the gauges. The front was down to about 90 while the rear was up at 110 or higher. It recovered, and over the next hour until we reached our stopping place for the night we saw it drop lower than the rear a few times, but it always recovered, and sometimes rather quickly, just 2 or 3 seconds to jump back up. We've driven another 1,200 miles since then, always with an eye on those gauges. Both are definitely lower than they used to be while traveling, but never drop below 90. It wasn't until we've been stopped and parked for a week now that we are noticing how quickly everything is dropping overnight. It doesn't seem that one is dropping faster than the other overnight.

Re: Air leak for beginners

Reply #6 – June 09, 2021, 05:05:48 pm

The coach shouldn't drop even if the gauges are reading 0. Maybe your leak is in the leveling system
or the check valve for the leveling system is shot. I don't know what pressure is needed to keep the
coach at ride height.

Re: Air leak for beginners

Reply #7 – June 09, 2021, 05:11:23 pm

Quote from: Dan & Annie – June 09, 2021, 04:51:00 pm

I noticed something happen a few days ago that I had never seen before. My wife was driving and I glanced at the gauges. The front was down to about 90 while the rear was up at 110 or higher. It recovered, and over the next hour until we reached our stopping place for the night we saw it drop lower than the rear a few times, but it always recovered, and sometimes rather quickly, just 2 or 3 seconds to jump back up.

Do you have an air throttle using air?

Re: Air leak for beginners

Reply #8 – June 09, 2021, 05:25:47 pm

I replaced both my pressure transmitters because of rapid drop and recovery. Was just the transmitters.

https://www.summitracing.com/parts/sww-82894

Scott

Re: Air leak for beginners

Reply #9 – June 09, 2021, 05:39:20 pm

You noticed your air dropped to 90# when driving and then went back up. The 90# is the "cut in" pressure of the D2 air pressure governor controlling the engine driven air compressor as you go down the road. That D2 also causes the loud air release back below the air dryer at the "cut out" pressure, when the pressures is built back up to 120-125# again after the D2 causes the air compressor to re-start due to the low 90#.

Re: Air leak for beginners

Reply #10 – June 09, 2021, 05:46:12 pm

Quote from: oldguy – June 09, 2021, 05:05:48 pm

The coach shouldn't drop even if the gauges are reading 0. Maybe your leak is in the leveling system
or the check valve for the leveling system is shot. I don't know what pressure is needed to keep the
coach at ride height.

Do you mean, "shouldn't drop" while running, or shouldn't drop once the coach achieves ride height? Mine drops to bottom overnight.

Re: Air leak for beginners

Reply #11 – June 09, 2021, 05:51:54 pm

Quote from: Old Toolmaker – June 09, 2021, 05:11:23 pm

Do you have an air throttle using air?

No, King Controls

Re: Air leak for beginners

Reply #12 – June 09, 2021, 05:56:42 pm

Quote from: RandallBrink – June 09, 2021, 05:46:12 pm

Do you mean, "shouldn't drop" while running, or shouldn't drop once the coach achieves ride height? Mine drops to bottom overnight.

There are two "drops".
1. First being the dash air pressure gauges for the front and rear brake system tanks. They are allowed to drop over time as the per DOT, and I think that total loss in 8 hours is OK. The engine is going to refill the tanks.
2. Second being the air bags going down, and allowing the coach to drop on to it's bump stops. Maybe just allow it to lean over. There the HWH system if it is "ON", should allow the computer to wake and check "level" about every 20 minutes, cause the electric 12 volt air pump to keep the coach floor level.
As mentioned in another message, there are 2 systems at play here. One is brakes, and parking brake, and the other is the air bag leveling system.

Re: Air leak for beginners

Reply #13 – June 09, 2021, 06:47:22 pm

Quote from: Dan & Annie – June 09, 2021, 04:36:30 pm

1. Where do I find an "air system diagram" for my coach?
2. how do I "verify the operation" of the check valves?

1. See Reply #4 posted by oldguy. May not be exact for your coach, but should be close. Better than nothin'.

2. On the diagram mentioned above, look for the small rectangular boxes with arrow inside. Some of them are labeled "I". Those are 1-way check valves. Two ways to verify their operation. You can remove it, take it apart, and see if it is clean inside. If full of trash, clean it out, or replace it. OR, you can test the check valve in place. Do this by running engine and getting air system up to full pressure. Shut down engine. Then open a drain valve or loosen a fitting UPSTREAM (before the check valve) and release pressure in the lines. If the check valve is working properly it will maintain pressure DOWN STREAM of the valve.

Example: The FRONT brake tank and the REAR brake tanks have a check valve at the inlet lines from the WET tank. We want to verify operation of these two check valves. Pressurize your air system. Read/record the pressure on both dash pressure gauges. Shut down engine. Open the wet tank drain valve and let it bleed down to atmospheric. Go look at the dash pressure gauges. Possible results:
1. Both gauges still show the same pressure as before. THIS IS GOOD! Both check valves are working.
2. One of the gauges is the same and the other one shows loss of pressure. One check valve has failed the test. Clean or replace it.
3. Both gauges show loss of pressure. NOT GOOD. Both check valves have failed the test. Clean or replace both valves.

Re: Air leak for beginners

Reply #14 – June 09, 2021, 07:21:42 pm

It is handy when chasing air leaks to be able to read system pressure at multiple points. A stock coach has only one way to monitor system pressure: the dash pressure gauges. These gauges show the pressure in the BRAKE tanks.

I added a pressure gauge in the engine compartment that reads WET TANK pressure. Tied into one of the unused ports on the D2 governor and mounted the gauge where I can see it through the engine hatch.

I also made up a little test manifold using a female air hose quick connect fitting and a pressure gauge. I put male quick connect fittings on both brake tank drain valves. With this setup I can slide under our coach and stick the pressure gauge on either brake tank to quickly check the pressure in that part of the system.

My test gauge can also be connected to my tire fill hose using a double male fitting adapter. On my coach the tire fill hose comes directly off the wet tank, so this provides another way of monitoring the wet tank pressure.

Re: Air leak for beginners

Reply #15 – June 09, 2021, 09:40:22 pm

So is it common for air brakes after motor shutdown to go from 110 to about 50-60 with two or three days, as my coach does?

Re: Air leak for beginners

Reply #16 – June 09, 2021, 09:49:55 pm

If your air stays up to 50 lbs. after two days you have a fairly tight air system.
I am happy if the air is up above 80 lbs. in one day.

Re: Air leak for beginners

Reply #17 – June 09, 2021, 11:17:48 pm

Quote from: dicksop – June 09, 2021, 09:40:22 pm

So is it common for air brakes after motor shutdown to go from 110 to about 50-60 with two or three days, as my coach does?

If your air pressure drops to 50-60 and stays there for some period of time, that is a sign the protection valves on your brake tanks are doing their job, AND also indicates that the inlet check valves on the brake tanks are holding. Air leaks out (someplace?) until the protection valves close, preventing further loss of pressure. With the protection valves closed and the inlet check valves preventing "backwards" escape of air, the brake tanks are essentially closed at both ends and should retain sufficient pressure to provide one or two air brake applications in an emergency situation.

It is also a sign that the location of your air pressure leak (or leaks) is most likely downstream of the protection valves.

See thread linked below for my feeble attempt at explaining how a protection valve works:

Air Tank Valves R&R

Re: Air leak for beginners

Reply #18 – June 11, 2021, 02:30:27 pm

Progress! Studied the schematics, blocked the coach up on safety stands, got under with spray bottle and here's what I found. I think this may be the protection valve at tank #2. Does that sound/look right? It does not appear to the the same BENDIX PR-2 that Chuck & Jeannie posted about, but it looks like it's in the same place.
Bad protection valve

Re: Air leak for beginners

Reply #19 – June 11, 2021, 02:49:44 pm

Make sure coach is on safety stands before you loosen that connection - raise coach, lower onto stands and make sure coach CAN NOT lower any further before you break that one loose -

good luck on the repair

there will be more leaks in the system - just too many connections - keep searching.....

:-)

Re: Air leak for beginners

Reply #20 – June 11, 2021, 04:51:59 pm

Quote from: Dan & Annie – June 11, 2021, 02:30:27 pm

I think this may be the protection valve at tank #2. Does that sound/look right?

Yes, I believe that is a protection valve. It may not be the same specific valve as on my coach, but it will serve the same purpose. If you can get a good close look at it from all sides, you may find a part number that you can Google search for replacement. If not, then the next best bet is to remove it and take it to a NAPA parts house. They will most likely have a replacement in stock. If not, they'll order it for you.

Be sure whatever valve you buy has a closing pressure (operating pressure) of about 65 psi or close to it.

The valve in your video looks like a KN31000 or a knock-off clone. The KN valve is no longer produced. Replacement is Haldex 90555399. See thread below:

diaphragm Valves

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