96 Silverado Z71 brake line replacement
 

I just bought a 1996 Silverado ext.cab Z71 4x4 with the 5.7 vortec. The brake pedal was going about halfway to the floor before braking. The front resevoir was empty,which supply's rear brakes, I put fluid in and tried to bleed, no fluid would pump. I replaced master cylinder and removed bleeder's one at a time and blew lines out with compressed air, lines were both clear. I then re-installed lines to master cylinder, filled resevoir & started bleeding rear brakes the pedal came up to a full pedal and then the brake line ruptured. It's ruptured in the truck frame right where the gas tank is and the frame gets narrow. I can't pin point the break, but appears to be the transition from steel line to flexable line. The steel lines from the back of the cab on back are rusted pretty bad. I guess my question is. It looks to me like you have to drop the gas tank to get to the lines to replace. Please tell me I'm wrong! I would be greatly thankful if someone would give me step by step instructions on removal & installation