I'd suspect a grounding problem for starters, when you cinch up the park brake you're probably affecting cab grounding - is your battery ground directly to the firewall as original, and another ground cable from the proper place under the toeboard to the engine?

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics