94 GTP, 3.4 no rear brake pressure

intermittant problem, stays for months goes away for weeks.Abs and check
engine comes light on.
When working rear calipers and pressure seem fine. 
Thought it was one of the two solenoids on the abs 
valve(sticking)but after looking at valve, each solenoid has a front and
rear brake line going to it and the front brakes work fine.
Any idea's on where to start.......thx

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