Welcome, I'm Chris (aka Moose) I'm an independent contractor here at Just Answer.
If you have good pads in the rear, rotors are not glazed, and no concerns with line restriction, caliper slide pins, or caliper pistons. Then that leaves the ABS and master cylinder to blame. There is a test for this but it requires using 2 pressure gauges threaded into the master cylinder. More likely to be a faulty master cylinder in your case.
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