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Jeremy, Certified Truck/Bus Technician
Category: Medium and Heavy Trucks
Satisfied Customers: 3380
Experience:  ASE Med/HD Truck Technician, NATEF Diesel Engines and Electrical/Electronics Technician, EPA 608/609 HVAC Certified, Steering/Suspension and ABS/Brakes Technician
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In the morning when I start the truck the secondary air

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In the morning when I start the truck the secondary air system will fill up fast but the primary won't charge until secondary is fully charged. After that everything seems to be normal. The truck is a 2007 Freightliner Century Class S/T and the VIN# *****

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That issue is caused by a sticking check valve in the air tanks on the secondary side. The aggravating part is in order to resolve it, the tank needs to be replaced because the valve is internal on the tanks. Thee is no external valve that controls that.

The tank needed is the wet/secondary tank. It has the check valve that is supposed to have equal pressure build between the tanks. Sometimes you can run air line cleaner or air line alcohol through the system to get debris out that would cause the tanks to do that, but in all the times I have had that issue, I have had maybe 1 out of 5 clear up that way. I had to replace more than not.


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Customer: replied 28 days ago.
Thank you Jeremy. I will need you to guide me through the process of changing the air thank. Which one is it ? Do you know the part number ? Thank you!
Customer: replied 28 days ago.
I mean is the one on the driver side or on passenger side ? Thanks

It is usually the tank on the passenger side causing it, but there are a few time the driver causes it. On the passenger tank, there is a double check valve on the line on the front that goes to the wet tank. Check that valve and make sure it isn't sticking to one side. If so, replace it. If not, then replace the tank for it is the internal one doing it. Passenger side tank.