A few years ago Gene brought his 64 Plymouth and asked me to change the brake fluid to Dot 5. I did and a month later he returned and ask me to change it back to Dot 3/4 because the breaks were to spongy. So back to Dot 3/4 and he has been happy for several years.
I think that in Cobras we need to change the fluid every three or four years.
Mike brought his Cobra in and wanted me to work on the brakes. I did a test drive and they were BAD. You could not stop that car in 300 ft (well maybe 100) but it was scary.
I took it back to the shop and changed the fluid. Then I could slide the tires. Never have I seen such a huge change in brakes.
I use Dot 3 or 4
synthetic brake fluid in all my cars.