You put in new brushes and it worked fine.
Then it began to cause MCB trips - either earth leakage or over current.
Somebody told you it might be the switch (very strange diagnosis - remove him from your Christmas card list)
You replaced the switch but it still cause trips - conclusion - you have not changed the fault by changing the switch - and if you can switch it on and off then the wiring of the switch must be fine - forget the switch entirely.
You do not say how often the machine caused the trip or at what point in the machine's operation.
Heating elements in machines like this are a common failure point - with no other information I would suspect the element.
I am not an appliance engineer - I know nothing - there's a chap in Lincoln New Zealand who is very expert - if he responds pay attention to what he says...
Good luck..., January 2015