A bad master will let the petal sink on a light pressure brake application as stated earlier. A hard heavy application will get the masters seals to work. This should be fairly easy to differentiate. Bench bleed the master if you decide to replace it. Or at the very least crack the masters fittings and allow the brake fluid to displace the air before moving the pedal
The ABS systems in these trucks trap air in them that can not be bled out by normal procedures, there is a ABS bleed routine to run with a scan tool. It cycles the ABS while bleeding. My Chevy 2005 3500HD needed to be bled this way after I installed a Stainless brake line kit ( lots of work) it also has the hydro-boost brakes. They use the power steering pump to servo assist the braking. Its been my experience that these hydro boost brakes can allow the pedal to nearly bottom when you stand on the petal. But at that point the ABS is in charge of the braking- not your brake pedal.
BTW, any Chevy truck of this vintage and used where it snows and salt is spread must have its brake lines looked at closely. They are notorious for failing under panic braking. The stainless kits are pricey but you will never need to replace them.
Greg
1987 Alfa Milano (Bellissima since 2008), Racing since 2008 Stafford Springs, 2nd overall 2011 NJMP, 4th at NHMS 2011, 2nd at Summit 2011, Into the wall hard at Stafford Springs 2011, 2nd at Monticello 2013, 3rd at NHMS 2013, 2nd at NHMS 2016. 2nd at NJMP 2018
25,000 racing miles in 32 races in 10 years. Yes its the same motor. Tell me again how Alfas suck? Update: Big moneyshift = new motor