dmg92001
Newbie
Short story: Safety first. Verify suspension, steering, brakes. Replace all fluids and filters (except for A/C refrigerant and desiccant, assuming the A/C works.) Flushing the brake fluid will require a scan tool to get all the old fluid out of the ABS system.
Everything rubber is suspect: Hoses, belt, tires, mounts, suspension bushings.
Then a proper and complete engine tune-up. Connect a scan tool, verify EVERY sensor and computer output. Look at fuel trims, misfire counts, and so forth.
There's multiple reasons for low brake pedal. You're right about the adjuster(s), and you'd want to remove the drums to verify. But that's just scratching the surface. Air anywhere in the system--including inside the ABS--is another possibility. Oversized (worn) drums. Weak brake hoses. Weak master cylinder--in particular the third chamber or it's check-valve.
Your truck likely has JB6 brakes, therefore low-drag calipers in front. This requires a Quick Take-Up master cylinder, and replacing that with a standard, but larger-piston master is popular but crazy.
I'm expecting that axle to be a 9.5" semi-float; G80 represents a Gov-Loc differential.
But the easy way to put this to bed, is to remove the differential cover, drain the fluid and take a look. Look at the gear ratio and gear condition while you're in there. Inspect the magnet for debris.
Thank you for all this detailed response - so from what I understand now, the two main types of differentials are open and limited slip, with gov-loc and posi being two different types of limited slip.
If I remove the cover and it is a gov-loc I will see this:
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and if It is a posi I will see this:
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And if it is gov-lock I use 80w90 gear oil with no additives
and if it is a posi, then I use the same oil with a friction modifier additive?
I assume I should not reuse the old oil/mix it with new- previous owner replaced the cover after it rotted out and said he used whatever GM recommended (and I have no idea what that is exactly).
I will definitely need to a lot more research into working on the brake lines (most I've ever done is bled them after changing a caliper).
I have a basic scanner but I don't think it will do what you mentioned about flushing the ABS.
An electronic tester indicated the brake fluid was bad so I'll definitely be flushing it soon.
I am changing the front shocks today (hopefully) and will open the rear diff cover this weekend.