Mechanical issues with the ABS HCU are rare, but they can happen. Valve seats and solenoid valves can stick or remain open due to contaminated brake fluid contains rust and sediment.

Contaminants may also become trapped in the ABS modulator and valves. Debris can become stuck or not seat proper due to debris, corrosion or contaminated brake fluid. Also, due copper particles in the fluid from the corrosion of brazing in the brake lines can plate the seats of the valves. This is due to the dissimilar metals that make up the seat, valve and copper ions in the brake fluid. The three act as the cathode, anode and electrolyte depositing the copper on the valve seat of the HCU.

If the inlet/isolation valve is stuck open, it will not affect normal braking in any way. It will only hurt the ABS system. This could lead to a pulling condition during ABS activation. If an outlet/dump valve is stuck open in one circuit, this could cause a pull condition during normal braking. This is due to the loss of brake pressure at a wheel. Typically, this is not discovered until brake hoses, calipers and other parts have been replaced.