Were they "gradually losing power" over a long period of time, or just during the round of use when they died?
Either way, checking the brushes as Bill suggests should be easy to do yourself. Relacing them should be a simple job if that turns out to be the problem.
Only snag you might run into is where to order replacement ones - not sure what you have in your area for power tool/electric motor repair places.
'75 GMC "Blood, Sweat & Gear$"