I'm having a spongy rear brake issue right now.
Had to replace the rear master cylinder/reservoir and brake peg after an accident. I've bled my rear for hours using a "mitey vac" as well as just some aquarium tubing and an 8mm wrench but to no avail. I also tried to force bleed the brakes from the caliper up to the reservoir without success. Went through two containers of DOT4 with minor results, and then I tried the TID (tie it down) method for 24 hrs. (seen in the tech section of this forum). No results. Dealer says they have a tool to properly bleed the rear brakes but won't say what it is, as they want to do the work (expensive!). I know how a Scouts rear brake should feel like, as my wife also rides a Scout.
I've been bleeding brakes on my own cars, trucks, and motorcycles since 1970 and have never had an issue like this.