Brake Fluid DOT 3 or DOT 5

[WarrenD]

I seem to recall a number of years ago that DOT 3 was the recommended brake fluid. At an MV show a few weeks back, many of the M37 guys said to use DOT 5. As I'm about to replace a wheel cylinder and hose anyway, this seems like a good time to switch to DOT 5 as long as it won't harm anything.
Thanks!

[just me]

Dot 3 is cheap and perfectly suited to drum brakes. It entrains water and needs to be flushed every few years. It is also an excellent paint remover.

Dot 5 is expensive and overkill for drum brakes. Although it does not absorb water, it entrains air. This can make bleeding difficult and the pedal feel spongy. It is inert to paint.

The 2 types CAN NOT be mixed. If you are changing from one type to the other you must completely purge the entire brake system of the old fluid. Easier said than done.

If it were me, I would continue to run whatever it has in it.

[WarrenD]

Thanks, kinda what I was thinking. Probably why I stuck with DOT 3 to begin with.

[ashyers]

I prefer DOT 3/4 for the reasons just me mentioned and some others.

One issue with DOT 5 is that if any moisture does get into the system it will collect and not disperse into the brake fluid like a DOT 3/4. This will lead to localized corrosion. If you use DOT 3/4 and flush the system every couple of years I doubt you will ever have an issue caused by moisture with a street car fluid. When you get into the really high temperature stuff the shelf life drops as it's very hygroscopic. It works great, but you need to bleed the brakes frequently because it sucks up moisture and the boiling point plummets. Silicone fluid is also slightly compressible leading to crappy pedal feel.

Andy

[Vaughn2]

I use Pentosin Dot4 in my PW and have been very satisfied with its performance. I no longer experience WC leakage from corrosion in the cylinder and cylinders don't lock up if the vehicle sits for extended periods. It's not cheap, but it's good.

[https://crpautomotive.com/brands/pentosin/brake-fluid/]

[Cal_Gary]

I run DOT 3.
Gary

[Rdinatal]

I run DOT 3.
Gary

Same here.