I'm not familiar with the brake system on the big blocks and heavier duty brake setups, but this definitely applies to the 1500s. For much of the '80s and '90s, GM had the brilliant idea to go with a low-drag caliper design in an effort to reduce fuel consumption. It wasn't very effective, and it had the knock on effect of giving GMT400s in particular the terrible brakes that they are known for.
The low-drag calipers use a different angle cut into the caliper seal groove. These seals are square cross section. When the brakes are appled, the angle allows the seal to flex outwards before sliding across the piston surface, and when the brake pressure is released, the seal wants to become square again and pulls the piston back in further. In theory, this keeps the pads off the rotors and reduces unwanted drag. It also means that the pads have to move significantly before they can engage the rotor when the brakes are applied, much like the brake pad knock back many of us have experienced on the track and/or with failing wheel bearings. On the GMT400s I've experienced, you can actually feel the similar effect by double-pumping the brake pedal rapidly during a stop. The second pump will give you a nice high, firm pedal...but only until the calipers retract again.
To allow the brakes to quickly cover this gap, GM came up with a step-bore Quick Take Up (QTU) master cylinder with a pressure-limited bypass valve. In any case, this master cylinder has a large diameter piston that causes the calipers to close quickly, and once brake line pressure rises, the bypass closes and allows the smaller piston to provide the actual mechanical advantage for braking force. All of this still takes pedal travel, though. This system works best when you slowly apply the pedal until pressure ramps up, and then add force to do your stopping.
Panic stopping, though, is a big problem with this setup. I've never understood exactly how GM was okay with it. When you are in a panic situation and suddenly stomp the pedal, the pressure spike in the master cylinder closes that bypass valve and immediately uses the smaller diameter piston....which takes much more travel to take up the gap in those low-drag calipers! This is part of why GMT400s do that scary "whoosh" thing with no actual stopping when you stomp on the brake pedal. If your system is in good shape, you will still have brakes but with a very low pedal. If there is any air, degraded rubber lines, etc, you probably won't have enough brake force to trigger ABS on dry pavement.
The NBS master cylinder upgrade is a good one because it is a non-stepped master cylinder and doesn't have all the QTU bits. The diameter conveniently splits the difference between the QTU piston and the actual brake force piston, so you get some dead travel but a firmer brake pedal overall.
Not all GMT400s have low-drag brakes. I believe the 3500s and 4500s had conventional brakes, and others may as well. I don't know much about the Hydroboost systems on these. Additionally, 9C1/Z56 police package GMT400s got a different brake system that uses the standard 1500 rotors, 80mm non-low-drag version of the 2500 calipers, and the larger 2500 drum brakes and 1 3/16" wheel cylinders. To go along with this, they got a special master cylinder that is identical in size to the 1.25" NBS master....but still has a QTU valve in it. That valve appears to do nothing in this application aside from allowing some initial pedal travel before firming up. Police departments must have had issues with their Tahoes that were supposed to replace the 9C1 Caprice sedans, and so GM had to do something for them. Interestingly, the Tahoe Limited got these good brakes as well.
I have a "normal" GMT400, K1500 Silverado with the crap QTU system. Even with all new parts aside from the booster, it is pretty wimpy. If I'm towing any significant weight, I double pump the pedal almost every time I stop just out of habit to make sure the pedal is there if I need it.
I also have a Tahoe Limited, with mostly old brake parts that still work fine. That brake system is actually really good. Stops well, good firm pedal, etc. It's what all GMT400s should have been. The only problem I have with it is that 20 minutes of enthusiastic driving on curvy roads will start giving me brake fade. The drums in the back are quite large, but the bias is all in the front because of stock brake dive, so they don't do much and instead dump all of the heat in the small front rotors. I don't really have brake dive anymore, so I need to figure out how to adjust the bias a bit and see if I can find some more aggressive rear shoes.
