I just completely replaced the entire brake system on a truck that had been sitting for over 20 years. I started by trying to bleed the brakes. I could not get an fluid to flow from the wheel cylinder bleeder valves. I did open the brake line at the T on the rear axle and had good brake fluid flow. I discovered the 20+ year old brake fluid had actually gelled. It was about the consistency of Vaseline. I did not have to change the brakes lines but I did have to rebuild or replace all 4 wheel cylinders and the master cylinder.

Dave from Northern Kentucky
My 54 3100