dot 3 vs dot 5

[Lloyd]

Putting on new power master cylinder. Manufacturer recommends dot 5. The system has been using Dot 3. They recommend flush with alcohol and install DOT 5. I've been reading pros and cons of both. Opinions flush and go with dot5 or just stay with dot3/4
LLoyd

 

[Rick Muck- Mark IV]

Not a fan of DOT 5 (silicone) for most applications.

Dry boiling point Wet boiling point
DOT 3 205 °C (401 °F) 140 °C (284 °F)
DOT 4 230 °C (446 °F) 155 °C (311 °F)
DOT 5 260 °C (500 °F) 180 °C (356 °F)
DOT 5.1 270 °C (518 °F) 190 °C (374 °F)

5.1 is glycol based and will not require a flush, it would be my choice.

 

[Lloyd]

thanks Rick
i'll check into dot 5.1 really didn't want to flush the system. I really don't see how to get all of the alcohol out of the system with out disassembling everything.
Lloyd

 

[Cliff Beer]

The vast majority of master cylinders have seals which are compatible with DOT 3 and DOT 4 and DOT 5.1 (all glycol based). As Rick suggested, I'd go with 5.1 (or 4) rather than DOT 5.

 

[Lloyd]

looks like dot 5.1 it is
thanks
LLoyd

 

[Eric A.]

I have read that dot 5 will expand when it gets hot and so could lock your brakes under some circumstances. Probably OK for just street use but not generally recommended for any kind of race use.

 

[Bob Putnam]

The sponginess problem with silicone-based (DOT 5) fluid comes from its higher solubility of air. When the fluid heats up, the air comes out of solution, making the pedal softer. When using silicone fluid, it's always a good idea to cook it to near its boiling point for a period of time, then be very gentle when handling it.

Even when fluid expands, as long as the linkage is set up correctly and the return port is open, the brakes shouldn't spontaneously lock up.

 

[Jim Rosenthal]

So what is the advantage to DOT5 brake fluid? It doesn't seem to have anything to recommend it.

 

[Jack]

It won't remove paint.

 

[mark b.]

and it doesn't absorb water, so you don't have to replace it on a regular basis....., what harley recommends for their motorcycles.

 

[Jim Rosenthal]

Got it. I suppose that's an advantage, but if the car is serviced regularly it seems less of a plus to me. Maybe it's better to replace it every year or two and know that the brake system is seen to.....

 

[René]

you could also use a good (race) Dot 4
like say Motul 600 or 660 (or from castrol and others)

Very high boiling point (325°C / 617°F), superior to conventional DOT5.1 non silicone base / DOT5
silicone base fluids (260°C / 500°F mini) and DOT4 (230°C / 446°F mini)

see pdf were the others mentioned are also in the 'test'>>

http://www.motul.com/system/product...ts/2636/rbf_660_factory_line_8472_(gb)_1_.pdf

I use it also in my Hartge, a bit overkill perhaps, but better that then boiling etc.

RBF660 = 100% Synthetic Racing Fluid – DOT 4
Very high boiling point : 325°C / 617°F
For hydraulic actuated brake and clutch systems