Chris
You have me worried
It tripled the RCd when I touched it .
Ok let's go though it one step at a time
Make sure the earth lead ( machine to 13 a plug ) is sound test with meter by thi I mean test from the metal parts of the mill to the earth pin .
If that's ok the you need to check or have checked the earth conductor in the socket , back though to the house consumer unit , because it sounds to me that it's not correct .
The above is very important and needs to be done
Now on to the brushes you remove the plastic cap and the brush should just pull out nicely , quite free as you know the new size you can make an assessment on that , they should not be chipped but look like they fit the commutator nicely
Chris take care
Your situation is why I am of the old school with bonding , I have a 10 MM. earth conductor connected to all the machines independent from the power cables an this is bonded back to a 2 metre earth rod that is also bonded back to the consumer unit in the workshop , this is then bonded back to the house consumer unit and then to another earth rod
As I had acces to the test gear to prove all this I know it's ok the reason for all this is to make sure that all the metal you can touch with both hands ( eg across your heart ) is at the same potitential
Run the motor off load and see what happens but until you are sure about the earthing put som marigolds on

Stuart
Chris
If you do not have one of these I recommend yo get one they are not expensive
http://www.amazon.co.uk/13-Amp-Plug-Socket-Tester/dp/B004BLPIBKOther site do have them as well well worth a check round now and then for peace of mind