TOPIC:

I just flushed my brake lines with new DOT 3 fluid only to find out that the master cylinder has a leak. Instead of rebuilding the master cylinder, I bought a 'newer' used MC off of ebay, but it calls for DOT 4 instead of DOT 3! D'oh! Well, I've heard that 3 & 4 are very similar and I'm wondering how thoroughly I need to flush the DOT 3 fluid out of the brake lines? Can I just 'push' the DOT 3 fluid out by adding DOT 4 to the master cylinder and pump until I think I have all of the DOT 3 out of the lines? How would you do it?

Thanks in advance!

Maverick wrote:

...Can I just 'push' the DOT 3 fluid out by adding DOT 4 to the master cylinder and pump until I think I have all of the DOT 3 out of the lines? How would you do it?

YES. That method is just fine. Same for the routine "flushes" without requiring re-bleeding from scratch.

I use a rubber squeeze syringe to remove most of the existing reservoir fluid, but leaving the bottom ports covered with fluid so as to avoid air entering the line. Then fill with new fresh fluid, pump it through (always leaving the bottom ports covered with fluid so as to avoid air entering the line) and repeat refilling and pumping until the new fresh fluid exits the bleeder valve.

Good Luck!

Patton is absolutely correct. You can convert from DOT 3 to DOT 4 without a major overhaul but not the other way around. The major differences is the boiling point. 400 degrees F for the DOT 3 and 500 for the DOT 4 if they are uncontaminated. (If I remember correctly)

Realistically you can use DOT 3 or 4 in either system. Dot 4 only has a higher boiling point because it is "drier". Brake fluid being hydroscopic inherently wants to absorb water. Dot 3 and 4 will both do this if not sealed tight. You CAN switch back and forth no problem, but if mixed you can only expect the performance of a DOT 3 fluid. The only thing NEVER to do is mix DOT 5 fluid with either 3 or 4 fluid. DOT 5 is a silicon based fluid that WILL NOT mix with the petroleum based DOT 3 and 4.