Alternator charging issue

[muncher]

Hi All. I have been enjoying many trouble free miles until last week.
I noticed the battery/charge gauge reading on the dash went from 13ish down to 9ish. So when I got home I removed the alternator opened her up and saw the brushes were almost worn down. I ordered a new set and a new voltage regulator at the same time. Put it all back together and still not charging, same as before.
I don't have a voltage reader to check the charge rate across the battery, buying one tomorrow. I have just been using the gauge as a guide.
Anyone out there ideas on what I can check next? Stumped me a bit this one.
Its a 1987 GMC Sierra 305 tbi engine.
Cheers