A few issues

[Hammerhead]

If the Jakes have a small delay, or "take a second or so" to "build pressure" (I'm referring to hydraulic pressure, not boost pressure) or doesn't sound like every cylinder engages at the same time, it is very possible that;
a)the detent holes in the valve rocker shafts are worn
b)the check valves in the Jake rockers are worn
c)a combination of both
d)the springs are weak in the Jake rocker arms are too weak, MOST LIKELY NON ELECTRICAL CULPRIT
e)all of the above

When the Jakes are activated, oil flows through the rocker shafts and into the Jake rockers via these check valves. When they get worn, or when the springs get weak, it allows some oil leakage delaying Jake engagement of one or multiple cylinders.

Read this post below, copied from another thread, and read the link where I discuss in detail the procedure as I learned about this issue myself, with a little help from the resident experts along the way...

(08-10-2016 )Hammerhead Wrote:  Can you more clearly define the knocking sound?
When you say "goes away when the Jake is on, are you sure or is it being masked by the noise of the Jake?
Do your Jakes "instant on full pressure" or do they "build pressure in a second or so"?

I reffered to mine as a "tapping" that also went away when the Jake was on. The reason mine went away when the Jake was engaged was because it was worn detent holes in the rocker shafts. The check valves were also worn, and the Jake springs had compressed a little over 1/4" at about the same mileage. What happens is the Jake rocker does not get held into the detent position and bounces (taps) off the cam.
This is why I believe we should be changing out the springs in the Jake rockers every 200,000 (when most guys run an overhead adjust). They just get weak over time and lead to wear, and I'm convinced that keeping good springs will extend the life of both the check valves and the rocker shafts.

edited to add...this is the link to my corrective action to my sound.
http://rawze.com/forums/showthread.php?tid=557
Key posts are;
6- shows the old and new shafts so you can see the wear on the detent hole
8- advise right from Mr Hagg about the springs, cause I dint know enough to go that far at that time. When you get to that mans shop, shake his hand and say Thank You for me please
14- this is the part number lists for the shaft (front and rear), check valves, and springs (for my engine)
17- an exploded view of one of the shafts and pieces.
18- old spring beside a new one to see the compression over time reduction of spring length
25- a complete list of the results of all the work I did, including the Jake service. I still stand by the statement of "You will not know how badly your Jakes have degraded over time until you complete this service"

Good luck, and keep us informed...