The system has to be completely flushed, correct? Is it true once Dot 3 is in a system, Dot 5 can't be used due to incompatibility with seals? Or is it Dot 5 needs special components? My 70S sits for 6 months of the year. Thoughts or experiences are welcome.

 

I've swapped to DOT 5 without doing anything special and had no problems. Today I use DOT 5.1, which is compatible with 3.0 and 4.0.

 

I changed to DOT 5 when I rebuilt my brake system about 15 years ago. That was a good time, as all the DOT 3 was out of the system. When the lines were disconnected, I flushed them with water and then isopropyl alcohol, and used compressed air a few times to make sure they were dry. I used stock seals.

Bleeding was the hardest thing to do. DOT 5 is prone to getting aerated easily.

I tried vacuum bleeding, then pressure bleeding, but finally realized that any rapid movement of the DOT 5 causes air bubbles.

I eventually settled on gravity bleeding. The fronts bled in a few hours, but the rears took a while longer. I raised the front end a few inches to give more pressure, and bleeding completed in about 4 more hours.

I put a clear tube on the bleed screws, so I could see if all the air was out.

I've never had any problem in the subsequent years, except for a minor leak at the distribution tee on the rear axle. It's easier to add an ounce every few years than to put a copper crush washer on the leaky joint. And that also makes sure I remember to check the MC level.

 

Yeah, I like the non moisture absorbing of the Dot 5. I don't see any advantages for Dot 5.1. What do you mean by nothing special? I had a bitch of a time when I put new Nicopper lines on the back and needed to use a pressure bleeder. There are fairly new wheel cylinders all on all 4 corners.

 

Brake fluids were deliberately designed to absorb water. Humidity will get into the system and water puddles won't perform well.

 

Good point, I was thinking of that as well. I may just leave it with DOT 3, it has pretty fresh fluid.

 

If, as you say, the car sits for long periods, you might consider DOT 5 for its non-hygroscopic properties. I have it in my summer-only cars and have had no problems with aeration or air bubbles.