I have run Dot 5 in my 37 truck for 20 years and have never had a problem. This past fall, I reworked the brakes just because it had been 20 years. I ran some new lines and upgraded to a dual master cylinder. Refilled the system, bleed the brakes and have a good hard, tire squeeling pedal. I think Dot 5 is the best stuff out there and you probably just need to bleed the system some more. Somtimes it takes more bleeding than what you might think. The great thing is that doesn't eat up the paint and the silicone kind of gives you a chance to clean up the under side of the truck. BTW, a good place to buy Dot 5 is a Harley store. They use that stuff all the time!
Joe
