Dot 3 VS Dot 5

IMO there is no reason to use DOT 5 silicone other than it doesn't eat paint but a shitload of reasons not to. The disadvantages so heavily outweigh the sole advantage that it simply isn't worth the risk to your life and your vehicle. Use Castrol LMA or DOT 5.1 and just be careful on the very rare occasions you may have to top up or flush the system....

 

Weasel's post above hits the nail on the head.

1) DOT 5 and DOT 5.1 are completely different and not compatible with each other.
2) DOT 5 is for trailer queens.
3) DOT 5.1 is the good stuff
4) DOT 4 is good enough for most people

DOT 5 has got a lot of problems for vehicles that are actually driven - permanently spongy pedal because you can't get all the air out, any moisture that does get into the system will puddle at low points because it will not be adsorbed into the fluid and carried out. Not HAMB-material but because of viscosity and compressibility issues DOT 5 is also frowned upon in ABS applications.

There's a lot of readily-available DOT 4 and DOT 5.1 fluids these days. Castrol LMA and Valvoline Synthetic, the ATE fluids, probably others equally good, are more than good enough for anything short of serious lapping of Laguna Seca, and you can find them just about anywhere. Not much reason to consider anything else for a street car.

If you are hardcore enough to need something more than 5.1 then you already know about Motul and AP high-temp fluids and Castrol $S$R$F and the relative benefits and drawbacks of each.

I'll note that the German mfrs, at least, spec a two-year brake-fluid change interval. In reality it works out to 'flush out your brakes when you change pads' and it's a good idea for any car. By the time brake fluid's been in the car for six months you may as well assume you're down to the 'wet' boiling point anyway.

 

DON'T USE DOT 5 DON'T USE DDOT 5 DON'T USE DOT 5

I had so many problems bleeding my brakes with Dot 5 and I just don't like the pedal feel. Once you put silicone in the system you can never change back unless you change every component

 

I have used DOT5 on all my racecars and older restored cars for over 30 years.
1. It does not mess with rubber.
2. It does not cause leaks.
3. It has a high boiling point.
4. It will not mix with other fluids.
5. It does make a softer pedal.
6. It will not hurt your paint.
I have found that folks who say it caused them problems had the problems before they installed DOT5. It is wise to flush the system with isopropal alcohol before switching over or better yet do a total rebuild before the change.
Problems I have found when using it. If the flex lines are old they sometimes react with DOT3, or 4. They swell and implode pinching off flow to and from the brakes. Using DOT5 which is thicker will in fact make the brakes not bleed pressure back through the pinched line. The lines did not swell as a result of the 5 they were already imploded and the thinner 3 would kinda bleed back...at least till the whole line was shut then there was no pressure to that axle. Guys blame the 5 for the evil results of 3 or 4.
I always at least replace the flex lines before a switch to 5. If it is a customer car they get a total rebuild and never have to worry about it.
Bleeding 5 requires that you not pump the crap out of the pedal like my dad taught me, instead open a line and push the pedal smoothy down then close the valve and repeat till the bubbles are out. A vaccuum bleeder is even better.Yes there have been times(very rare) when it was hard to get a bubble out...but I have always been able to do it.
I have heard all the horror stories listed on this subject before...many times and they all were not caused by the DOT 5. Install it right and you will have no problems.

 

Air is not generated within the fluid. Air already suspended in the fluid eventually coalesces to form bubbles. The effect on pedal sponginess remains unchanged.

The minute differences in compressibility between one fluid and another is not something a driver will notice with their foot. Silicone may feel spongier due to its tendency to keep air suspended in the fluid.

Realize that your favorable experience does not necessarily mean that all DOT 5 is safe for all seal materials. I am aware of numerous cases where new parts, in a clean new system, installed by a competent individual, were attacked. No it doesn't happen in all cases, but it does happen.