DOT 3 vs DOT 4?

[DLK]

My master cylinder reservoir is a little low and I do not know which fluid the prior owner used. I am quite certain it is not silicone (DOT 5). Can anyone tell me the difference between 3 and 4 and which you would recommend I add? I plan to bleed the system so will probably need more than enough to refill it.

[rustyzman]

You can top off DOT3 or DOT4 with DOT4 and be just fine. You should not top off DOT4 with DOT3 as this lowers the boiling point of the DOT4 fluid. If in doubt, put in DOT4. Do Not mix DOT3 or DOT4 with DOT5. DOT5 is not compatible with conventional brake fluids.

The main difference between DOT3 and DOT4 lies in the boiling point of the fluid. DOT4 can sustain higher brake temperatures without boiling.

If really in doubt, bleed out the brakes with new fluid of your choice. Really should change brake fluid about every 2 years anyway due to moisture absorption.

Edited June 1, 2010 by rustyzman

[Robert Horne]

I read somewhere, that the fluid turning dark is where the fluid has absorbed moisture.

[1955 plymouth]

DOT 5 is a silicone base, not a Glycol base as the DOT3 or 4. the o-rings and seals used for DOT5 are different than the o-rings and seals used in DOT3 systems. The DOT4 has a shelf life of about 2 years as far as I know the DOT3 does not have this crucial shelf life..

Edited June 1, 2010 by 1955 plymouth

[Dodgeb4ya]

As stated in the above article DOT 4 has a higher boiling point (30-50 degrees) than dot 3. Really should use it over DOT 3.

[DLK]

You can top off DOT3 or DOT4 with DOT4 and be just fine. You should not top off DOT4 with DOT3 as this lowers the boiling point of the DOT4 fluid. If in doubt, put in DOT4. Do Not mix DOT3 or DOT4 with DOT5. DOT5 is not compatible with conventional brake fluids.

The main difference between DOT3 and DOT4 lies in the boiling point of the fluid. DOT4 can sustain higher brake temperatures without boiling.

If really in doubt, bleed out the brakes with new fluid of your choice. Really should change brake fluid about every 2 years anyway due to moisture absorption.

In my prior 53 DeSoto I put in all new wheel cylinders and master cylinder so switched to DOT 5 after flushing the lines. From reading old posts on this forum it sounds like I cannot switch to DOT 5 in my Dodge without replacing all the rubber parts which I am reluctant to do since my brakes seem to work well with no visible leaks. I appreciate the suggestions I use DOT 4 rather than DOT 3. I had an old opened can of DOT 3 but will dispose of that now.

[rustyzman]

Good call on throwing out the opened can of brake fluid. Brake fluid is hygroscopic and will pull moisture from the air right through the seal on a bottle cap. My own rule is to buy small bottles and dispose of partial bottles that are not used within a couple of days.

These systems tend to experience the majority of their corrosion problems from the inside out (being collector vehicles not driven in salt or water). Not using old fluid and regularly flushing systems is cheap insurance.

[Mike@DRH]

Brake fluid is hygroscopic and will pull moisture from the air right through the seal on a bottle cap.

Really ??

[rustyzman]

Really. To clarify though, the seal on the bottle I am referring to is the paper one on the cap, not the foil one from sealed at the factory. The foil one should be just fine.