One problem related to brake differential proportional valve has been reported for the 2001 GMC Sierra. The most recently reported issues are listed below.
I took my truck in for the state inspection. . . Kept failing due to brakes after changing the brakes it failed on the e-brake, that we have never used, when checked the was no e-brake pad on one side and you can see where it has worn, the cable does not even tighten like it is supposed to. . . For something we do not even use I do not understand how it can be this bad. I still have the e-brake we changed on my 2001 GMC Sierra.