Auto Locking Hub Problems

[HighOCTANE_HSV]

Hey all, new guy here

I'm new to owning a Hilux and a 4x4, and I'm having an issue with my truck. The auto-locking hubs won't engage. I've done a bit of a diagnosis and found that the button on the high/low range selector works, the relay works and I've pulled the electrical connector out of the front of the steering rack/transfer case area and when I push the tip in of the connecter (pretending to lock the hubs together) the light in the dash will indicate that the 4x4 system is engaged. So the problem area is in and around the vacuum diaphragm itself. Has anybody had any experience with repairing these parts or would be able to help me out? Everything in the transfer case is all good too. If you need more info or some photos I can post them.

My model is a 1998 Hilux 4x4 SR5 LN167R

Thanks guys