Grove rt-60S, "Jerky" throttle operation

My Grove RT-60S with a Cummins V8 diesel has a pneumatically operated throttle/speed control.

The way it works is that it is a pneumatic cylinder with springs that actuates the lever of the injection pump.

Pressure is needed to retract it, so as the crane starts cold, it runs a bit faster for a minute until pressure builds up, the cylinder retracts, and idle speed slows down.

It is operated (extended) by pressure applied to another port, which causes it to extend forward.

The problem is that it operates in a "Jerky" manner: as I depress the foot pedal or engage throttle, nothing happens for a while, and then the engine suddenly revs up.

This is very annoying, especially when trying to drive steadily.

I did some investigation.

First of all, the cylinder itself operates smoothly when placed on my bench and operated by shop air.

Second, I tested air pressure by removing the throttle pneumatic cylinder and putting pressure gauges on the retract and extend pneumatic hoses that operate this pneumatic throttle. What I found out is that the air pressure acts "jerky", does not rise at first, and then the needle suddenly jumps to 20 PSI.

I am sure that this is the cause of jerky engine operation.

The question is what causes it.

There is two pneumatic controls in the cab, the "gas pedal", as well as the throttle lever. Both could be operated at the operator's convenience.

Both of them, when so operated, cause this behavior, so my suspicion is that there is something else.

There has to be a relay valve that takes input from both of these controls, and generates air pressure to the throttle.

So far I have not found it, does anyone know how it works and where it might be located?

In the cab somewhere? What does it do?

Thanks!!!

Grove rt-60S, "Jerky" throttle operation