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Not comprehending exactly what you are saying there. Four new tyres but the problem was there before new tyres?
What condition were the old tyres? Any chance that they had mismatches (wear or pressures) that could have started this and that it's just carried over to the new tyres?
old set of tires were all same size with same tread depth, only reason for replacement was dryrot
Being an 88 K1500 it will only have RWAL so the back doesn't know what the fronts are doing.
I'm not arguing with you, but how does the system know the rear wheels are under-rotating without a reference speed from the front wheels?
"When the ABS control module detects a difference in the average speed of the rear wheels compared to the vehicle's overall speed, it initiates antilock braking." https://www.aa1car.com/library/abs_kelseyhayes_rwal.htm has a very good explanation of how the Kelsey-Hayes RWAL system works.
Replaced a year ago doesn't mean much these days with parts quality issues. Wouldn't hurt to give everything the once over while you're there, Most times an out of balance tire, loose steering components, broken belt in a tire, bent wheel will give a shimmy or shake in the steering wheel. Not what's being described. Pulsating pedal usually leads right back to the brakes, abs unit, dragging frictions cooking cast iron. run out that was present to begin with that got worse, etc. Most pro mechanics I know do a minimal skim on new out of the box parts, drums and rotors, before they even go on. No comebacks and they don't self-destruct in short order. Rotors, drums, and frictions are not where you want to buy the cheapest crap you can get your hands on. Buy pro grade stuff. Agree with Schurkey, If the rear's aren't working right it's not hard to cook the fronts.
