It’s the most help I can give, seeing as I have no idea what Seafoam is.
Seafoam is a brand of automotive chemicals marketed in the States. They sell various additives, cleaners,and chemicals. It's primarily a retail brand. A lot of guys over here like it.
As far as your comment anyone that posts about a non Triumph should expect to receive a bit of stuff in the feedback. I can just hear what would come back on a HD Forum about a TRIUMPH question.
Not to get political here I respect Vet's. Spent some time around the VA in the mid 60's to mid 70's. Saw a lot of Fd up guys.
As far as your current problem. As you are in a VOC compliant state your aerosol brake cleaner will contain acetone. You might want to try a few shots of this to see how the gunk reacts. Another cleaner to try would be something like the Castrol Purple cleaner. Be aware it probably contains Butyl and is nasty stuff be sure to protect the areas where any liquid is entering or leaving the tank
"THE WORLDS PRE-EMINENT MOTORCYCLE"