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Topic: Air Pressure When Braking (Read 985 times) previous topic - next topic

Air Pressure When Braking

We just took a short (560 mile) week long trip in our coach after having been away from it for several months.  I noticed an odd phenomenon with the air pressure gauges.  I don't know if it is a new occurrence or if it has always been this way and I just never noticed it before.  (I have only owned this coach for fifteen year and over 90,000 miles.) 

When I apply the brakes, the lower gauge (rear brakes ?) immediately drops ten to fifteen pounds lower than the upper gauge.  I always had sufficient pressure, it never got down to where the alarm dinged.  The pressure always built right back up so I was not overly concerned but it didn't seem right.  Also, I noticed that when I applied the retarder, the lower gauge would immediately drop five to ten pounds. 

Is there something I should be checking in my system or is this rather normal? 

Thanks,

Richard
Jan & Richard Witt
1999 U-320  36ft WTFE
Build Number: 5478 Motorcade: 16599
2011 Jeep Wrangler Unlimited w/Air Force One
Jan: NO5U, Richard:KA5RIW
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Re: Air Pressure When Braking

Reply #1 –
Could be a leaky brake chamber. Chalk the wheels release the brake should be no sound of air leaking from chambers, then apply the brakes with the foot pedal same thing no sound of air leaking coming from the chambers.
2001 Foretravel U320.

Re: Air Pressure When Braking

Reply #2 –
Short story test.
Build pressure .
Shut down engine for 5 min. Should maintain air minus 10# or less.
 Start engine , apply brake pedal (park brake off)
Should maintain air for 2 min MOL with foot on brake.
Any great air Loss exceeding these values should get fixed. 
  There are better details and values in u tube. But these are my minimum values .

Re: Air Pressure When Braking

Reply #3 –
After Mike's first test, I would build back to cut-out PSI and then shot off the engine.

Apply the service brakes.  After initial drop due to applying the brakes you should loose no more than 2-3 PSI in 5 minutes. There is a DOT standard for the maximum permitted PSI drop, but your coach with air system in good condition should be able to do this.

If excessive, with safety stands in place have someone else push on the brake while you listen for the leak and/or spray suspected areas with kiddie bubble solution and watch for the bubbles.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Air Pressure When Braking

Reply #4 –
Step #7 is ambiguous.  Step on the brakes firmly and note the pressures.  From this point forward the air pressure cannot drop more than 4 PSI.
1992 Foretravel Grand Villa
U225 SBID Build No. 4134
1986 Rockwood Driftwood
1968 S.I.A.T.A. Spring
1962 Studebaker Lark
1986 Honda VF700C
1983 Honda VF750C
Charlie, the Dog was broken out of jail 24 Oct 2023
N1RPN
AA1OH (H)e who must be obeyed.

Re: Air Pressure When Braking

Reply #5 –
Quote from: Old Toolmaker –
Step #7 is ambiguous.  Step on the brakes firmly and note the pressures.  From this point forward the air pressure cannot drop more than 4 PSI.

That is 1 minute time. The pressure will keep dropping depending on leaks other than the cans while holding longer than 1 minute. Had Chip's climb on my fuel tank on the crane and watch my gauges during the test.
1999 40 ft. U-320 wtfe build 5563 Chuck & Lynda's "Rollin' Inn"  2030 watts solar
prev. mh's 71 GMC 5 yrs. 73 Pace Setter 1 yr. 78 Vogue 5 yrs 81 FTX 40ft all electric 18 yrs. 1996 Monaco Signature 3 yrs.
2014 Grand Cherokee Overland
Dream as if you will live forever. Live as if you will die today.  James Dean

Re: Air Pressure When Braking

Reply #6 –
Following up, I ran the tests above and the coach passed the DOT test but was well outside Brett's tolerances.  I could not hear any obvious hissing leaks but did not do the bubble test yet.  Unfortunately further investigation will have to wait a while as I am putting the coach in storage for a few months. 

Richard
Jan & Richard Witt
1999 U-320  36ft WTFE
Build Number: 5478 Motorcade: 16599
2011 Jeep Wrangler Unlimited w/Air Force One
Jan: NO5U, Richard:KA5RIW
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Re: Air Pressure When Braking

Reply #7 –
Off pedal leaks vs on  pedal leaks. 
Pushing brake pedal isolates that section from the rest.
You chase each section until you just settle for good enough and safe .

Re: Air Pressure When Braking

Reply #8 –
If you do the leak tests above and find no leaks, the brake stroke could be getting out of adjustment. The increased travel of the chamber  (Bad slack adjuster?) will use more air, usually more noticeable with repeated brake usage coming down a hill, stop and go traffic.
2001 Foretravel U320.

Re: Air Pressure When Braking

Reply #9 –
My coach has been storage for the past four months and I am just getting back to address my air brake situation.  I started the main engine and the pressure built up to 115 pounds.  I shut off the engine and ran the DOT brake test.  The system passed with flying colors, no perceptible pressure drop on either of the two checks.  I pumped the brake pedal as prescribed and the warning light and sounder came on as the pressure passed through about 70psi and the parking brake knob popped out at about 25psi. 

I then restarted the engine and the pressure would not build at all, it remained at about 25psi.  I shut the engine off and left it for a short while then restarted the engine and again the pressure would not build at all.

My question:  Would this be an indication of a defective D-2 valve or an air pump not functioning? 

I have a spare D-2 valve that I could throw on if there is a chance that that is the problem. 

Any thoughts?

Richard
Jan & Richard Witt
1999 U-320  36ft WTFE
Build Number: 5478 Motorcade: 16599
2011 Jeep Wrangler Unlimited w/Air Force One
Jan: NO5U, Richard:KA5RIW
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Re: Air Pressure When Braking

Reply #10 –
Last year my dryer malfunctioned and I couldn't build up any air. To rule that out put you hand
below the purge valve to see it any air is come out.
Peter and Frieda Morin
1999 36ft. U320 Foretravel
Build # 5436
1998 Suzuki Sidekick Sport

Re: Air Pressure When Braking

Reply #11 –
Quote from: Jan & Richard –

I then restarted the engine and the pressure would not build at all, it remained at about 25psi.  I shut the engine off and left it for a short while then restarted the engine and again the pressure would not build at all.

My question:  Would this be an indication of a defective D-2 valve or an air pump not functioning? 

I have a spare D-2 valve that I could throw on if there is a chance that that is the problem. 

Any thoughts?

Richard

Air Pressure won't build
Doug W.
96 36' U270 CSGI #4946
04 Toyota Tacoma 4x4
PNW

Re: Air Pressure When Braking

Reply #12 –
If you did not feel any air doing test in post # 10, then take a hammer and give the D2 a love tap.  If compressor starts. Replace D2
2014 ih45  (4th Foretravel owned)
 1997 36' U295 Sold in 2020, owned for 19 years
  U240 36' Sold to insurance company after melting in garage fire
    33' Foretravel on Dodge Chassis  Sold very long time ago

Re: Air Pressure When Braking

Reply #13 –
Quote from: turbojack –
If you did not feel any air doing test in post # 10, then take a hammer and give the D2 a love tap.  If compressor starts. Replace D2
The correct term is Percussive Maintenance, not "love tap.".
1992 Foretravel Grand Villa
U225 SBID Build No. 4134
1986 Rockwood Driftwood
1968 S.I.A.T.A. Spring
1962 Studebaker Lark
1986 Honda VF700C
1983 Honda VF750C
Charlie, the Dog was broken out of jail 24 Oct 2023
N1RPN
AA1OH (H)e who must be obeyed.

Re: Air Pressure When Braking

Reply #14 –
If you dont hear an air leak while running...
The D2 gets air from the  line from the main /wet tank to control the shuttle valve. I believe the inlet that is  furthest from the adjuster nut,  is the tank input line. 
The  D2 shuttle valve controls the compressor  unloader valves.  That air circuit is near the center of the D2 . The center circuit also is the purge line , that goes to the purge valve .  Small air line near the purge on dryer.    The line can plug up and keep pressure on the unloader.  Blow through the line  to check .
  If your hearing is good, you may be able to hear the unloader valves jingling, if they are unloaded. You can unhook the center input line at the D2 , (or dryer end) to depressurize the unloader. ( and build pressure) Be careful of course .    This line should not have pressure until the D2 shuttle valve moves and opens the unloader /purge ports.
 It sounds like you may have a stuck unloader or D2 . IMHO.    Ijust went through  all of this.
 
 There are check valves on the inlet and outlet of the dryer , or possibly on the lines/tanks,  feeding the dryer.  The check valves keep the pressure up while shut off and depressurize the compressor head between air charge cycles. If your system holds pressure for 4 or more hrs, I doubt that the check valves are in play.

Re: Air Pressure When Braking

Reply #15 –
Quote from: Jan & Richard –
When I apply the brakes, the lower gauge (rear brakes ?) immediately drops ten to fifteen pounds lower than the upper gauge.  I always had sufficient pressure, it never got down to where the alarm dinged.  The pressure always built right back up so I was not overly concerned but it didn't seem right.  Also, I noticed that when I applied the retarder, the lower gauge would immediately drop five to ten pounds. 

Is there something I should be checking in my system or is this rather normal? 

Thanks,

Richard

Richard going back to your original issue, rapid pressure drop. I've had this three times. All three were do to a failing pressure transmitter
https://www.summitracing.com/parts/SWW-82894
Would also pop up at same rate. Not building D2 or dryer failure
Scott

Re: Air Pressure When Braking

Reply #16 –
For my follow up:  I tried a percussive adjustment on the D-2 air governor.  I was not really very aggressive with it.  I am a bit reluctant to hammer the side of my diesel engine.  Anyway, that had no effect on my air pressure not building.  So I dug out my (new) spare D-2 and installed it.  Super easy job, accomplished entirely from inside the coach under the bed.  That appears to be where the problem was.  All is well now and I am ordering another spare D-2.  Thanks to all for the suggestions. 

Richard
Jan & Richard Witt
1999 U-320  36ft WTFE
Build Number: 5478 Motorcade: 16599
2011 Jeep Wrangler Unlimited w/Air Force One
Jan: NO5U, Richard:KA5RIW
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