2020 T680 cabin air bags

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Not the best video.
Cab leveling valve looks something like this with an arm on it.

 

A man has a 2020 Kenworth T680. His cabin air bags deflate every time he hooks up to a trailer. The air bags stay deflated for several hours and eventually inflate. Are there some valves for the cabin air bags under the frame?



Yes, there are valves involved in the cabin air suspension system of a 2020 Kenworth T680, and they could be contributing to the issue you're describing.

Here's a breakdown of what's likely going on:
✅ 1. Cab Air Suspension System Overview

• The cab air suspension uses air bags (air springs) to cushion the cab and improve ride comfort.
  
  
• These air bags are filled and regulated by a ride height control valve (or leveling valve), which typically senses the cab position and adjusts air accordingly.
  
  
• The system is connected to the truck’s main air supply, which also serves the brakes and trailer.

Problem Description

• Cab air bags deflate when hooking up to a trailer.
  
  
• They stay deflated for several hours before slowly inflating.

This suggests a priority issue in air distribution or a malfunctioning valve.

️ Possible Causes and Checks:
1. Cab Ride Height Valve (Located under the cab or frame near air bags)

• These valves regulate air to the cab air bags.
  
  
• If faulty or sticky, they can fail to refill the bags promptly.
  
  
• On a T680, it’s typically mounted under the frame on the driver side, near the rear cab mount.

Check: Inspect or replace the ride height control valve. Look for slow or no response when the cab sags and air becomes available again.

2. Air Supply Priority

• When connecting a trailer, the system prioritizes brake reservoirs.
  
  
• If the cab air system doesn’t have a dedicated tank or check valve, it may be last to get air.
  
  
• A leaky valve or faulty pressure protection valve could cause delay.

Check: Ensure there’s no restriction or leak in the air line from the primary supply to the cab suspension valve. Also verify if a pressure protection valve (usually on the supply tank) is working properly.

3. Air Dryer and Pressure Build-up

• If the air dryer purge valve sticks, it may slow air build-up after trailer hookup.
  
  
• This could delay all non-essential air systems like the cab suspension.

Check: Watch your air gauges. If both primary and secondary tanks fill normally, but the cab still stays deflated for hours, the issue is downstream — likely the ride height valve or plumbing.

✅ Summary
Yes, valves under the frame, especially the cab ride height valve, are involved and are likely culprits in this case. Have a mechanic:

• Inspect or replace the cab leveling valve.
  
  
• Check for air supply routing issues or restrictions.
  
  
• Make sure the air tanks and valves prioritize cab suspension properly after trailer hookup.

 

I remember a time when that #### wasn't all tied into everything on a truck. I can't even wrap my head around what possible reason there'd ever be to even deflate cab air bags when hooking to a trailer.

 

Why they would put anything electrical between the air supply and valve is beyond me!! Who thinks this crap up?

 

That is strange for sure considering it hasn't always been that way probably with the bags deflating on the cab. Is the leveling valve for the bags something you can easily reach and manually deflate or inflate the cab bags by moving the control valve "arm"? I suppose even if it is accessible and you did that, it could just go back down after you manually inflated them anyway. Good deal though that you discovered how to make them inflate otherwise.