Percussion??????
Can you elaborate........
And yes a brush and a lot of solvents especially stuff like Hoppes #9 and nitro solvents will not touch copper.
If it does'nt claim to be a copper solvent it is'nt, 9 times out of 10.
To me it sounds like you've got something bigger than a copper issue...but start with the easy fixes first then move to the more difficult ones.
Step by step trouble shooting.
Let us know how this progresses.