For info, I pulled the ABS unit the other day (to get better access to the firewall opening for sealing up the LS1 ECU harness) and was surprised that hardly a drop of brake fluid came out. It was really an easy job except for the bitchy electrical connectors that didn't want to separate, no surprise on a 12 year old car I guess.

I'll know in a few weeks if R&R of the ABS unit requires extensive brake system bleeding to regain a firm brake pedal.

 

You're going to have to spend some time bleeding the abs unit. Just crack open the lines with the compression fittings one at a time just like you do with the bleeders on the calipers.