Brake fuid question

[JSharpe]

I just finished a C5 brake upgrade on my 71 camaro. I put prestone synthetic DOT 3 brake fluid in it. My master cylinder started leaking, so I am swaping it out. Can I put non-synthetic DOT 3 back in and let it mix with the synthetic? Or should I keep using the synthetic DOT 3? Or, could I flush the system with DOT 4 and just use that? Sorry for all the questions, I appreciate the information. Thanks, Jonathan

[MarkM66]

I believe it's all technically synthetic. So any DOT 3 will work.

[FrankZ]

I think the important thing to note is to never mix conventional brake fluid with silicone base brake fluid.If it is a matter of switching from say dot3 to dot 4 just give it an extra bleed or two to flush out the lower grade.

[Apogee]

DOT3/4/5.1 fluids are all typically compatible, however the fluid manufacturers will usually warn you not to mix higher grade fluids with lower grade fluids since it will compromise the temperature rating of the fluids to something in between the two. Dont' get met wrong, it will still be higher than the DOT3 rating, just not at the DOT4 level if you mix the two.

DOT3/4 fluids are readily available everywhere, so I'd suggest running the higher grade of the two and calling it a day if you mostly just see street use. A normal bleed/flush is never a bad thing and should be done at least every 2 years to keep the fluid fresh and remove water from the system.

If you're tracking the car, you could possibly benefit from DOT 5.1 fluids, however the added expense and lesser availability is a drawback for some. There's not a lot of need to run 5.1 if you don't fade the brakes with DOT3 or 4 fluids as it doesn't gain you anything except maybe piece of mind (and possibly a little better pedal feel depending on the compressibility factor of the fluid).

Tobin
KORE3

[SLO_Z28]

DOT 4 is more hygroscopic than DOT3. Thats why in police pursuit vehicles they use only high quality DOT3 fluid. Water will absorb through the rubber in the brake lines, and the heat of high performance driving will cause it to boil and cause sever brake fade, even complete loss of pedal. Also after every pursuit or every 30,000 miles they bleed the fluid.

Its a good idea to bleed your fluid after a track day. I know its probably overkill but I bleed my fluid every time I change my oil, I do this every six months. With a pressure bleeder it really doesn't take that long, and rotate my tires anyways so its not much more work.

For a good fluid Castrol GT LMA is a high quality fluid and is readily available at most parts stores.