The brake pedal in SBC cars is all but completely divorced from the braking hydraulics. What you feel in the pedal is a simulated pedal feel and not actually feedback from the brakes. Only at the very end of travel is a small portion of very unassisted emergency braking pressure.

A sinking brake pedal is a symptom of a failing SBC module. I've had this exact complaint on several cars and not been able to duplicate it nor had it leave a fault code trail. They've all ended up needing the SBC replaced. I'd think yours should still be under warranty. You should go back in a huff, make a stink, and have them replace it. Maybe even discreetly mark yours to keep them honest.

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90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff