Motor tripping circuit breaker

Hi,
I hope someone may be able to offer some help.
I have a cleaning machine with 2 large vac motors ( 1400 watts ).
One of them runs fine and smooth for a few minutes and then trips the house circuit breaker. I have removed the motor, took the carbon brushes out, cleaned it all and put it back together, still the same.
I then wired it directly to a plug top to rule out the machines wiring and yet again the same problem.
It appears to have had little use by the look of it, the bearings run very freely and smooth and there is no commutator sparking or arking and there are no smells from it.

Thanks in advance.

Hello
Is it defin the circuit breaker that trips or is it the RCD ?
What size is the circuit breaker?

It's the large RCD that trips.

There is no way a visible inspection is going to rule out an insulation problem. You need expensive meters to really test it. I have in the past working on the Falklands where buying a new motor was not an option used special varnish after drying and cured earth problems. But I would say since the motors are easy to buy and replace that simple method is to replace.