Question about back brakes

I noticed once in awhile my back brakes seems like they don't grab for a split second. Makes a slight sound and acts as though it slips when applying brakes. Anyone know what this is and should I be concerned. It has only done it a few times and not every time I ride. Thanks for your help

when's the last time you bled it?
i would recommend that you swap the fluids. sounds like could be air in the line somewhere that just floats around.
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