Detroit Series 60 acting strangely (12.7 450)

Company I currently work for has two Sterling L9500s with Detroit 60s. Both 12.7s, I can't remember exact power rating right now, but i'm sure one is 450 and the other is a 435? Anyway, the one I drive has been a headache. I can't get any proper answers from anyone, and it's not a problem I can replicate, as it's totally random.

My issue is the truck is almost impossible to downshift at times. It's an 8LL truck, with Jakes. It's more common when the truck is loaded vs unloaded, but when i'm downshifting, at random times, but mostly in high range between 8 to 7, and 6-5, sometimes 5-4, the trucks RPMS hang while my foot is to the floor. I managed to time it once, with my foot buried, from 1200 rpm to 1800-2000, it took 5 seconds to reach the higher RPMs. It's unpredictable, some days it doesn't do it but once, others it's constant while loaded. The other Sterling doesn't do this, but it hung slightly ONCE.

Because of this sometimes when I go to downshift, expecting a normal RPM climb, I end up grinding the piss out of the next gear. To best describe it, it's like the jakes are on, but they are switched off. It's the same as that delay you get when shifting up or down with jakes while you wait for them to deactivate. Something usually within a millisecond, but it does it over the course of a couple of seconds.

I've been told two theories, that the valves are so far out of adjustment it's hanging it up. Also that the jake system is attempting to engage while off, and that's causing it, or the jakes are getting hung up while on/off.

Any help would be appreciated, I haven't found anyone with this exact or even similar problem on all the posts i've searched.

 

Didn't mean to double post this. Oops

 

I completely forgot about the pedal. I’ll see about getting that checked out.

Last time it was in for this issue Freightliner claimed it was a CAC leak. Which I didn’t believe. Charged $4500 and fixed basically nothing.