I have fitted new brushes by removing just the end cap that they are fitted in - the brushes are held in by plastic barbs on the end which spring open when the brushes are pushed fully home, which is why they are well nigh impossible to pull out without removing the end cap.
If you remove the end cap you should then be able to squeeze the barbs together and push the brushes out - then if you refit the end cap first you should be able to push the new brushes fully home and they should stay put, fingers crossed,
Hope this helps
kamztrad