Q: Hydraulic Modulator on 1999 Volvo S70

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How can you tell when your hydraulic Modulator is going bad?
(1) Answer
If you're talking about your ABS Modulator, then I suspect your ABS light is on and there's a code for the ABS Pump Motor? If so, this is a control unit issue. The unit can be rebuilt or replaced to address this.
If I'm off base here, provide some more info so I can better advise you.