Gravity bleeding brakes is one of those old-school gearhead tricks that seems to have gotten lost in the modern world. Maybe it's because kids these days just don't know how to sit back and wait -- maybe it's just because people don't trust anything that isn't expensive or fast. But GMC owners are a different breed, and as likely to try gravity bleeding as any GMC owner for the last 100 years.
Gravity bleeding -- opening all four brake bleeders and the master cylinder lid, and keeping the reservoir full for an hour -- isn't just an alternative process. For some cars, like older Corvettes, it's the preferred one. Gravity bleeding is fairly fool-proof on most vehicles, provided that you're only bleeding air from the brake lines after the ABS modulator, and that the ABS modulator never ran dry. It will work on almost any vehicle, as long as the brake fluid reservoir is higher than the brake calipers; the further up it is, the faster the gravity bleeding goes. That's not a problem with the tall GMC, where the brake reservoir is a couple of feet higher than the calipers.
So, yes, you can gravity bleed a GMC, with or without antilock brakes. But there's a caveat: There's no way you'll get fresh fluid into all of the channels in the ABS modulator, master cylinder, or accumulator without pumping the brakes. So gravity bleeding isn't appropriate if there's air in the modulator or master cylinder, or if you want to completely flush all of the old fluid out. For that, you'll need to do it by the method GM recommends. But if you're just looking to bleed air from the lines or calipers further down, gravity bleeding will work.
