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
1 answer
Pull drums and check for leaking wheel cylinder or other contamination on brake shoes. Is brake fluid at 'max' in M/C reservoir?