Wow, so now we dont push the pedal all the way down when bleeding brakes? WTF....thats news to me. Since the primary piston pushes the secondary piston you need fully depress the pedal to get full travel of both pistons during bleeding, this is in on car OR bench bleeding procedures. As far as corrosion in the master damaging the seal and casuing a mushy pedal....not likely, its much more likely that the crappy old fluid in the calipers was forced back quickly into the lines and BPMV during brake service, aerating it. Id do a full bleed again, and in the proper RR, LF, LR, RF order. if that doesnt work get a hold of a Tech2 and do a BPMV bleed.
Oh BTW I didnt bother to read any of your shadetree links but this is straight from the factory Service Manual:
Bench bleeding clinder
# Secure the mounting flange of the brake master cylinder in a bench vise so that the rear of the primary piston is accessible.
# Remove the master cylinder reservoir cap and diaphragm.
# Install suitable fittings to the master cylinder ports that match the type of flare seat required and also provide for hose attachment.
# Install transparent hoses to the fittings installed to the master cylinder ports, then route the hoses into the master cylinder reservoir.
# Fill the master cylinder reservoir to at least the half-way point with GM approved brake fluid from a clean, sealed brake fluid container. Refer to Fluid and Lubricant Recommendations.
# Ensure that the ends of the transparent hoses running into the master cylinder reservoir are fully submerged in the brake fluid.
# Using a smooth, round-ended tool, depress and release the primary piston as far as it will travel, a depth of about 25 mm (1 in), several times. Observe the flow of fluid coming from the ports.
Manual bleeding vehicle:
# Submerge the open end of the transparent hose into a transparent container partially filled with GM approved or equivalent DOT-3 brake fluid from a clean, sealed brake fluid container.
# Have an assistant slowly depress the brake pedal fully and maintain steady pressure on the pedal.
# Loosen the bleeder valve to purge air from the wheel hydraulic circuit.