I'm pretty sure the word "fin" is a typo and should read "in the hydraulic circuit".
If the engine idle RPM is too low, and there is no "idle up" vacuum dashpot on the mixer throttle lever to advance the throttle when the engine RPM lowers.........it pretty much a sure thing that the engine will be choked out on these active hydraulic power brakes systems in the 5F series trucks.
Lots folks have had to retro fit idle up dashpots on 5F models that have Impco LP equipment.