There is no magnet in the monovalve. There is a solenoid for valve actuation. Does not sound like the monovalve is your problem since power is needed to shut off flow.
Put your hand on the in/out hoses of the monovalve. If they are both hot, monovalve is not the problem.
How will the solenoid actuate when there is no magnet?
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'85 300D Cal 280,000 miles
'14 GLK 350 60000 miles