What brake fluid was used. I think the 1800 calls for DOT4 but most others will not cause an issue except DOT5. Dot 5 is silicone based and is not good especially if it has ABS. Just because Honda did the change does not guarantee it was done right....unfortunately. A mechanic might have grabbed the wrong bottle. I know there are at least some Harleys that use DOT 5. In one particular situation the owner used DOT 4 and it melted all the seals, even the rubber on the reservoir cap.
Also and more likely look at the anti dive valves. The 1800 had so many issues that some owners cut the plungers. Read the links below.
http://gl1800riders.com/forums/show...roblem-what-has-Honda-done-to-solve-the-issue
http://powersports.honda.com/news/0...-safety-recall-notice-2001-2012-gl1800-a.aspx
Also and more likely look at the anti dive valves. The 1800 had so many issues that some owners cut the plungers. Read the links below.
http://gl1800riders.com/forums/show...roblem-what-has-Honda-done-to-solve-the-issue
http://powersports.honda.com/news/0...-safety-recall-notice-2001-2012-gl1800-a.aspx