When I was going through my brakes several years ago, I was persuaded to go to Dot 5 by my friend at the local Carpart Store. His reasoning was this: that dot 3 fluid absorbs moisture, these old brake systems are open vented systems, and that especially when these trucks are not being driven daily the brake fluid absorbs too much moisture and the workings will rust. He had gone to dot 5 fluid on all his old vehicles.
I am not a specialist in these questions, but if this line of reasoning is a pile of bovine excrement I would like to know, so I can prepare for another round fixing my brakes - which have been running for 2 years now on dot 5 fluid without problems.
