1991 gmc 1500 brakes

I HAVE REPLACED PADS ,ROTORS, CALIPERS,MASTER CYLINDER & STILL HAVE BRAKE LIGHT ON, NOT MUCH PEDAL,&NO BACK BRAKES.BRAKES HAVE BEEN BLED SEVERAL
TIMES.THE ONLY TIME I HAVE BACK BRAKES & A GOOD PEDAL IS WHEN IS OFF.ANY IDEAS?THANKS IN ADVANCE,
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only have brake pedal when engine is off is what i meant to say
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