what would cause 3 of 4 brakes to disengage during stop? on 1999 Toyota RAV4

When I depress brake all seem to engage for a few seconds then all seem to release and the right rear skids.

Asked by for the 1999 Toyota RAV4
Pull drums and check for leaking wheel cylinder or other contamination on brake shoes. Is brake fluid at 'max' in M/C reservoir?