Problem with Jake Brake

[g_man1146]

I have a series 60 Detroit with a three position Jake brake. Originally it was
wired to either be "Off" or come "On" with throttle lift. Blue Bird added a wire
to the brake light relay and changed the switch so it has a center "Off". One
way it works as before with throttle lift and in the other position the Jake
comes "On" when the brakes are activated.

Occasionally it stops working altogether and when this happens I pull the relays
in the compartment under the drivers window, spray the sockets with cleaner and
all is well for another year or so. (Bennie showed me this "fix")

The last time this happened only the "throttle lift" mode started working again.
The relays, wiring and switch check OK. The green LED next to the switch lights
when the service brakes are applied so I know the trigger voltage is there. Any
ideas on what to try. The Jake works normally in the "Throttle Lift" position
just not in the "On with Service Brake" position.

Thanks for your help!

Rich D. '99LXi43' CT