Brake Problem

1997 GMC Yukon SLT Nose diving bad when brakes applied. Taking up the rears helps for a day or so then back to the same nose-diving thing. Could my combination valve be stuck? How to check it?
Also have a bad/rusty ABS sensor as the ABS keeps activating when stopping from lo speed as in a parking lot. Are these problems related somehow? TIA.
Regards, JR

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