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Topic: Air pressure drop (Read 1035 times) previous topic - next topic

Re: Air pressure drop

Reply #25 –
Quote from: hdff –
So I brought the coach home today and had a chance to poke around on it. First I let the air build up to 110 then killed the motor and left it in travel mode. Checked it all day 110# on both gauges @ 9:30 and at 6:30 tonite it was 67# on both gauges, acceptable by fot standards. Then I soaped all the tanks,bags,leveling switches and fittings I could find.. no major leak and I could ... so why do the gauges go down so fast while going down the road? I have to take it to fot tomorrow and I'll monitor the gauges again...
When you are moving, hcv is constantly moving and  adding and bleeding off air. You may have one position that the valves leaks by. FOT can probably diagnose the problem pretty quickly.
Jim C.
coachfree, previous 1997, 1999, 2000, and 2003 Foretravels.

Re: Air pressure drop

Reply #26 –
Clearly, there is a leak (or leaks) somewhere. And if you lost 60 PSI on both gauges/tanks, you might have a leak in the dryer, and a bad check valve(s).

I say that, because of chance of exactly the same size leaks occurring on both independent systems is pretty low.
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 drop

Reply #27 –
Situation like this is when a pressure gauge on the wet tank is handy.  Comparing wet tank pressure to the other tank pressures can help determine which direction the air is going, and also help narrow down the possible leak locations.
1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"

Re: Air pressure drop

Reply #28 –
It would have to be on the charging circuit. Dryer, D2 relief valve .  A part/valve, compressor,  that runs when the engine runs. IMHO.  The static leak down will be the same regardless of compressor action. 

Re: Air pressure drop

Reply #29 –
But, for both tanks to leak down, the check valve(s) must be bad.
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 drop

Reply #30 –
Original problem was rapid cycling of compressor when travelling, slow leak down when stopped is another problem all together. In travel mode, only compressor, dryer and hcv's could cause that quick of a pressure loss, if not manifested when stopped. OP says these are not leaking when stopped.
Jim C.
coachfree, previous 1997, 1999, 2000, and 2003 Foretravels.

Re: Air pressure drop

Reply #31 –
I am having a similar issue but less severe, as my Ventura vacuum generator is only running non stop whenever the park brake is not "on". My oshkosh air schematics seem to be somewhat lacking info. I am no expert but I believe the park/emergency brake pnuematic logic is it requires air pressure to release the brake. That being said, a leak in that system can only be detected, with the wheels chocked, compressor running and the park/emergency brake in the released position.
Old Phart Phred, EIEIO
89 GV ored 36' #3405 300 hp cat 3208 ATAAC side radiator, mountain tamer exhaust brake

Re: Air pressure drop

Reply #32 –
The fact that both "front" and "rear" tanks simultaneously leak down to the same pressure (67 psi) might be due to faulty check valves on both tanks.  It could also be caused by a leak somewhere in the air system downstream of the protection valves, IF the leak is in a line or component served by both brake tanks.  To determine whether this is possible, one must study the air system schematic.  I am not familiar with the air system on the 2000 model U320, so I don't know if the two brake tanks are tied together anywhere downstream of the protection valves.

As a example, on our '93 U280, there are 1/2" lines coming off both brake tanks that are joined together, and then supply air pressure to the brake treadle valve (via line #26).  After the treadle valve, the air continues to a number of different brake components in both the front and the rear brake circuits.  A leak in any one of those components could drain both my "front" and "rear" brake tanks down to the pressure at which the protection valves closed.  This would happen around 50-60 psi on our coach, assuming both protection valves closed at the same pressure.

SO, what is more likely - having 2 faulty check valves, or having two protection valves that close at exactly the same pressure?
1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"

Re: Air pressure drop

Reply #33 –
Quote from: Chuck & Jeannie –
The fact that both "front" and "rear" tanks simultaneously leak down to the same pressure (67 psi) might be due to faulty check valves on both tanks.  It could also be caused by a leak somewhere in the air system downstream of the protection valves, IF the leak is in a line or component served by both brake tanks.  To determine whether this is possible, one must study the air system schematic.  I am not familiar with the air system on the 2000 model U320, so I don't know if the two brake tanks are tied together anywhere downstream of the protection valves.

As a example, on our '93 U280, there are 1/2" lines coming off both brake tanks that are joined together, and then supply air pressure to the treadle valve (via line #26).  After the treadle valve, the air continues to a number of different brake components in both the front and the rear brake circuits.  A leak in any one of those components could drain both my "front" and "rear" brake tanks down to the pressure at which the protection valves closed.  This would happen around 50-60 psi on our coach, assuming both protection valves closed at the same pressure.

SO, what is more likely - having 2 faulty check valves, or having two protection valves that close at exactly the same pressure?
Good point Chuck, but problem is rapid cycling of compressor when running. OP is not worried about leak down when braking or stationary, but when driving normally and not using brakes. His problem does not appear to involve brakes or protection valves. He just cycles between 90 and 120 psi too quickly, but only slow leak down when not running.
Jim C.
coachfree, previous 1997, 1999, 2000, and 2003 Foretravels.

Re: Air pressure drop

Reply #34 –
How about a leak in a brake component that leaks a lot of air when driving down the road, and the leaks less air when parked?  What is the brake component that is in different positions when parked and when driving?  When driving, full system air pressure is applied to the rear brake cans to release the parking brakes.  When parked, air pressure is released from those cans so the spring brakes can activate.  I don't know much about the rear brake components.  Is it possible to have a leak in one of the rear brake assemblies that would produce these symptoms?
1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"

Re: Air pressure drop

Reply #35 –
Quote from: Chuck & Jeannie –
How about a leak in a brake component that leaks a lot of air when driving down the road, and the leaks less air when parked?  What is the brake component that is in different positions when parked and when driving?  When driving, full system air pressure is applied to the rear brake cans to release the parking brakes.  When parked, air pressure is released from those cans so the spring brakes can activate.  I don't know much about the rear brake components.  Is it possible to have a leak in one of the rear brake assemblies that would produce these symptoms?
A possibility, OP could park coach and set parking brake. If no leaking from release cans, that eliminates that possibility,
This is one of those problems you do not figure out, you just eliminate everything it could not be.
Jim C.
coachfree, previous 1997, 1999, 2000, and 2003 Foretravels.

Re: Air pressure drop

Reply #36 –
Quote from: Old phart phred –
I am having a similar issue but less severe, as my Ventura vacuum generator is only running non stop whenever the park brake is not "on". My oshkosh air schematics seem to be somewhat lacking info. I am no expert but I believe the park/emergency brake pnuematic logic is it requires air pressure to release the brake. That being said, a leak in that system can only be detected, with the wheels chocked, compressor running and the park/emergency brake in the released position.
I believe this applies to all coaches running on the road.
Old Phart Phred, EIEIO
89 GV ored 36' #3405 300 hp cat 3208 ATAAC side radiator, mountain tamer exhaust brake
 

Re: Air pressure drop

Reply #37 –
Do not know about your coach but this is the old system.  Sold my 72 gmc conversion, rolled down the parking lot and took out a horse trailer, [no horses hurt]  I had warned him to chock wheels.
http://www.schoolbusfleet.com/forum/topic.asp?TOPIC_ID=5041
Jim C.
coachfree, previous 1997, 1999, 2000, and 2003 Foretravels.