379 bouncing bad

Had the same problem on my 2005 379. It's been riding rough and I could hear the air bags bottoming out when I hit bumps and such. Yesterday there was a grinding sound when I let off the accelerator. I first thought it was the carrier bearing, the power divider or maybe a U-joint. I pulled it over and started checking things out. Turns out, the leveling valve was allowing air to leave the air bags but not filling them back up as needed. Eventually, the bags got so low that it was throwing the drive line out of line causing the U-joints to make an unbelievable grinding sound.
Had I not taken the leveling arm loose and moved the arm up and down I would not have caught this problem. Turns out, that it has been an ongoing issue and the leveling valve has not been keeping the bags at the correct height which is where the rough ride and the bottoming out sound was coming from.

 

Out of curiosity, where do you feel the bouncing? In the steering wheel or in the seat?

 

Did you find the problem?
Have you checked your motor mounts? Strange as that sounds, put a jack under the back of the trans. and see how much travel it goes up and down. The trans. is mainly held by the bellhousing attached to the motormounts. There should be a sprind hanger near the back of the top of trans.with two longer bolts and springs to take up this bounce. If the motor mounts are worn or this rear bracket broken, will give a strange bounce/bang.
I don't think this is quite what your describing but I know this is a similiar problem and these mounts are so easily overlooked.

 

Have you replaced cab bushings? You will be amazed at what all starts working right after you replace them. Even if it does not fix your problem it will be the best 300.00 (Hunter Peterbilt Erie Pa parts and labor) you'll ever spend.

 

I finally solved my rough ride!!! Turns out it was a tire problem. I got rid of the cheap Chinese junk tires and put some good bridgestone tires on. This thing rides like a dream. It's smooth and quite. I can hear myself think.

 

wHEN YOU ADJUST IT DOES IT SUPPLY MORE AIR TO THE AIR BAGS?

 

The man says it also happens when he is parked at a dock with the forklift running in n out so that rules out motor mount.I think there are check valves in the air system.might not be letting the bags stay at the same PSI an keeping them balanced,cuz u said truck was sagging also in the rear.So maybe some are lower then the other side and making it bounce more, one side has less resistance then the other side and makes it give easy.just guessing but makes sense.to m anyway

 

What if the shock absorbers on the cab air suspension were bad? t would cause the cab to bounce.