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Brake fluid in brand new system: is it synthetic?
Ok, well I think I’m going to totally flush and refill with 5 and make sure I put that label somewhere..
Thanks everyone for the great advice!!
(10-06-2019, 12:41 PM)Rmach1 Wrote: Ok, well I think I’m going to totally flush and refill with 5 and make sure I put that label somewhere..
Thanks everyone for the great advice!!

Final question for you.  Did you verify what Bill had posted about the peroxide cured EPDM material needed for dot 5.  More than likely you are ok but I would double check the manufacturers web site to see if it states it or what the type of dot you can use.  If you find out that any part of the brake system will not take dot 5 I would flush with any of the other dots and use one of them. I would probably go with the 4 or 5.1


"The only dumb question is the one not asked"
[+] 1 user Likes Kilgon's post
Ahh ok... well I can’t remember what brand brake calipers and wheel cylinders are so I’m thinking it may be best to flush it like crazy and do 4
I don’t think I’d be able to determine if the runner parts have been treated
Thanks for the input!!
Back in 2007 I was finishing my 69 Mustang vert restomod and installed DOT 5 thinking it would be advantageous for all the usual reasons (resistant to water since it spends more time sitting than not, won't hurt paint if it develops leak, etc). It had a SSBC drum to disk conversion kit. In 2014 I needed to replace a defective booster and figured I might as well change the Master Cylinder at the same time. As I discussed the issues with LEED, they mentioned to me they no longer advocated DOT 5 since they had encountered so many system issues with seal failures. They basically said use of DOT 5 would void any warranty with the MC I purchased. I did not install the new MC at the time due to a mis-match in the line fittings.

I flushed my system 3 times with 90% isopropyl alcohol before installing new DOT 3/4 (has slight advantage over DOT 3 with water absorption) after installing the new booster. BUT I had to replace my rear wheel cylinders (2007) two years ago when the seals failed - found puddle of fluid under one wheel when taking out of winter storage. As I began to pressure bleed the system with the new wheel cylinders I was very surprised at the traces of purple fluid I kept seeing in the fluid I was getting out of the rear lines. So I re-flushed the entire system with an entire quart of new DOT 3/4. I also replaced all flex lines with new stainless wrapped lines a couple months after the wheel cylinder change and did another system flush at the time.

Now I'm in the process of finally changing the (2007) MC since the piston seals have failed causing the brakes to lose their hold after a few seconds of pedal pressure. I will be interested to see the condition of the seals in the old unit.

So my experience with DOT 5 has not worked out very well over time. My 73 vert always had Dot 3 or DOT 3/4 and there were never any issues with seals over 25 years & 30K miles of driving. I should note the 69 system only has 6K mileage since 2007.
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