When you do the test, touch the probes together. Remember this reading, this is the resistance of the test leads. Now touch the slip rings. Subtract the first reading from the second to get the actual ohms.
One of my meters reads .8 ohms touching the leads. If the slip rings read 5.9, then I subtract the .8 from the 5.9 and get 5.1. Yours can read different. My other meter reads .1 ohms touching the leads.
If you can't get the probe in the brush holes you can remove the brush holder, gives you more room. Adding extra wires can make it harder to get a good reading.