'90 K1500 Small Q's

(Just got it, much TIA):
- has aux batt pkg & isolator w/ring terminal; WTF does an aux batt do? Pos connector is only energized w/key "on" (and not in "accs'y"
or "off"). Is it wired stock to provide any juice for accessories through the truck's wiring? Or does it just charge off the 85A alt & sit there for added wiring or as a reserve spare?
- reasonably tight 4.3V6 w/4-spd (OD) auto & 3.42 gets 15MPG in careful driving. :-( Been offered free manifold for 4bbl. Think good tuning of a carb could outdo the TBI/ECM in economy? I doubt it but thought I'd ask those experienced.
- had the "mystery brake light" issue past 2 daze, went away with a batt disconect/reconnect (module reset); any typical "really wrong" causes to look for as to why it came on to begin with? This is the stock RALBS which is out in 4WD. Usuals (levels/pedal/parkbrake switch) all fine.
- FWIW have the leaky/corroded/shot PS pressure line & hv run pump dry, hv noted other posts here/other NG's about PS pump repl heartaches; a rebuilt pump is only $64 retail at local NAPA - wondering if others getting porked?
Cheers, Frank
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