Brake bleeding/proportioning valve reset

[87r10owner]

I have an 87 R10 that I swapped the rear-end out and it and the brake light on the dash stayed on! I did bleed them after and found that a wheel cylinder was leaking so I completely rebuilt all my brakes with all new parts. Once again I bled the brakes and it still stays on so the proportioning valve I'm guessing is stuck to one side. So how do I reset it and get that light off??? What it the rubber button on the side of it for?
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