I'm looking for a GM replacement master cylinder gasket. The rubber piece underneath the cover? I have found two at an auto parts site. The only thing that distinguishes them is that one is for a model with 11 inch rear brakes and the other is for 13 inch rear brakes? Does it matter which one I get? I am unsure about the rear drum size on my truck and am confused on which one to buy.

 

i shouldnt have to say this but, 1) get a tape measure and go do the obvious, 2) compare the two gaskets side by side, if you cant see any apparent difference then either one will work fine, my vote would be for the cheaper one at that point.