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Old 01-01-2016, 10:24 AM   #15
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Lots of experance in air brakes tells me the same thing.

The only exception is that some accessories ( air ride, horn, air seat ) run off a regulator, set to stop flow at around 60 lbs, to save the air for the brakes.

Even with that, it would need to be a big leak, to not climb above 75 lbs even at high engine speed.

It would also slow down the air buildup and never get to the air dryer purge pressure or cycling constantly.

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Old 01-01-2016, 10:39 PM   #16
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I hope that we will hear what the issue was when it is diagnosed and repaired.

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Old 01-02-2016, 04:59 AM   #17
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Originally Posted by dpinvidic View Post
Curious why folks suspect the dryer.
The wet tank is in front of the dryer, and the pri & 2nd tank are fed by the wet tank.
If the pri tank shows 120psi, that proves the dryer is ok and the wet tank must have 120psi.

In thinking about this more, it is not likely to be a leak, if it was leaking bad enough to prevent reaching 120, it would certainly drop to zero right after the engine was turned off.

A restriction would not prevent reaching 120, it would just take longer than the other tank.

It really sounds more like a sensor problem which could affect both the gauge and the alarm.

The OP really needs to put another gauge on the system to confirm the actual pressure before buying parts and guessing.

Just the opinion of an arm chair quarterback


Hi Dan,

I tend to agree with you, for all the reasons you mention. The ONLY thing about the entire system that is non-normal is the low reading on the rear tank. It fills, burps, responds to pedal applications, alarm status due to <65psi, etc.

One thing i have learned from this post is that some responses "attacked" me for placing lives in jeopardy bla bla..... As a recently retired captain for a major airline, I made decisions for decades that determined a real problem or a gauge, and I would like to assure them that I would never consider going without full brakes. But in an ideal world, both gauges would read identically. (I have also seen gauges that indicated a perfectly normal condition for a completely inoperative system)

And so now, I will research the sensor by verifying the actual system pressure.

Carolyn, Jim, and Clara Allen
'05 Newmar Dutch Star
Fisherville, KY
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Old 01-02-2016, 08:08 AM   #18
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Hi Jim,

Good to hear your on the right track. We are thinking that the actual pressure is good.

Here us a typical pressure warning switch.
60 psi switch

Looks like you have a Spartan chassis. Maybe someone can tell you exactly where that switch is.

My first guess would be that the warning switch and the gauge sensor are two different things. But this assumption is contradicted by the fact that a single breaking event caused the alarm. The odds are against two components being bad. So this suggests that you may have a pressure transducer which is used to drive the gauge, and trigger the alarm.

The first order of business would be to determine if this is true for your chassis.


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