Help needed

[kmk]

Check your brake lights and cruise operation. The brake switch code is likely caused by corrosion in the wiring. Start from the BCM connectors and the switch connectors, any "green crusties"... Make sure all of them are clean and the connector female contacts are not spread apart, make good contact. Check the resistance values on the j1939 terminating resistors at the test plug.
J4 Connector is for the Cab. there Shouldnt be anything on pin f11.
J5 has pinouts for J1939 datalink and zero volt reference, make sure its clean and makes good contact. Really important.
Check all of them.

ECM connectors too. Power and grounds too. You are likely chasing intermittent data link problems due to excessive resistance. Get yourself wiring diagrams.
Fuses and breakers too, any discoloration there just replace them. Make sure they make good contact too.

I would start at the BCM and test plug then chase the rest. See how things respond.
Post your results, so we can all learn from them.