can i bypass the brake lines going to the valves under the master cylinder, and hook up direct on a 91 gmc?

Are you eliminating the ABS?

Yes tenspeed just doing away with both valves and hooking direct to master cylinder.

If you are eliminating the ABS, you need to start from scratch including a new proportioning valve. I would go to a place like Stainless Steel Brakes and get a complete tubing set, including flex lines and proportioning valve and a new master cylinder, and start from scratch, since the system components you have are all designed to work with the rear ABS that was standard or the 4 wheel ABS that was optional.

Why would you want this? fear of high-pressure?

If the truck has the rear wheel only ABS and the electronic controls have failed, it may be IMPOSSIBLE to repair. I know someone needed the control box, about 3 years ago, and there were only 3 left then. Chances are a new one is unobtainium, which is the problem with a lot of the electronics in the more modern vehicles. Once the supply is gone, unless some company makes after market replacements, the finite supply runs out, and you have to jerry rig a way around it.

You already have a porportioning valve disconnect line and contact it to porportioning valve you bleed brakes done to shut off lite unplug wire on porportioning valve