Stock factory 93. Thirty years old. 120k on the clock. Who knows what the the PO’s did or didn’t do to the brake system. Cheap shocks and rotors confirmed. Wasted lower control arm bushings and way screwy caster alignment attempt confirmed. Otherwise babied creampuff showing typical 30 yr old ailments.
Can I purge the brake system with a DOT5 transfusion or will any caliper/master/abs/pressure seals go AWOL on me? Who’s done it? What happened?
Hi Rick …
Here’s an excerpt out of an article on the matter …
All brake fluid is synthetic , as in chemically made, but there tends to be an emphasis on calling silicon brake fluid as synthetic, and this does not mix with glycol based fluids. A few extremely top of the line engineered DOT 4 and 5.1 manufactured liquids can give a superior pedal.
All the old domestic stuff had seal materials that couldn’t handle silicon so rebuilding every brake component with modern seals was mandatory. Dunno about the British stuff, when or if they got that handled. As it is, needing a full complement of both SAE and Metric on the ground just to get around under the hood is a big enough PITA. Sometimes my wonderful old rides need to sit around for months or years and not having to rebuild every cylinder bore and piston in the hydraulic systems and constantly bleed the damn things is worth it to me. Thanks for reminding me about 5.1
To answer my own question:
Wikipedia:
DOT 5 is a silicone-based fluid and is separate from the series of DOT 2, 3, 4, 5.1. It is immiscible with water, and with other brake fluids, and must not be mixed with them. Systems can change fluid only after a complete system changeover, such as a total restoration.
It contains at least 70% by weight of a diorgano polysiloxane.[7] Unlike polyethylene glycol based fluids, DOT 5 is hydrophobic.[8] An advantage over other forms of brake fluid is that silicone has a more stable viscosity index over a wider temperature range. Another property is that it does not damage paint.[citation needed]
DOT 5 brake fluid is not compatible with anti-lock braking systems. DOT 5 fluid can aerate when the anti-lock brake system is activated. DOT 5 brake fluid absorbs a small amount of air requiring care when bleeding the system of air.[9]
Lack of acceptance of silicone-based fluids led to the development of DOT 5.1, a fluid giving the performance advantages of silicone, whilst retaining some familiarity and compatibility with the glycol ether fluids. DOT 5.1 is the non-silicone version of DOT 5, defined by FMVSS 116 as being less than 70% silicone. Above that threshold makes it DOT 5.
Not mentioned so far is the issue of increased compressibility of silicone-based fluids. Some argue that this results in a soft pedal.
I always use Dot 4. It’s full synthetic and unless you’re racing, is superior for a street car. It’s also cheap, and easy to find.
Greg …
Just a stupid question. Since all brake fluids are synthetic, when a brake fluid advertises itself as “full” synthetic what’s with that. Unlike motor oil that can be petroleum oil, a blend with synthetic, or full synthetic, aren’t all brake fluids always full synthetic ?
Because of the very complex ABS on my 1990 XJ40 I go with factory recommended DOT 4 and bleed the system every 3 years or so. All my vintage British car back to 1951 MGTD, 1965 3.8S, etc are running DOT 5 silicone quite successfully.
Based on my limited Internet research the short answer is ‘no’. DOT 2, while " not widely used in the automotive industry" is mineral oil (petroleum) based.