Hmmm ... Honestly, I have no idea what is going on. Glucose itself is neutral, and should have no effect on the pH.
If presented with your observations I would probably do the following:
1 - Check the integrity of my chemicals.
2 - Add glucose individually to the PBS, DMEM and HEPES and see if it's maybe reacting (!?) with something in one of them.
Is there glucose in your DMEM?