Geojerry

The low check brake fluid warning msg appeared this morning on my 91 S4.
Can I assume dot 5 synthetic is used by certiffied mech from the prev owner?

Geojerry

My owner's manual says dot 3 or 4. The engine reserve container shows ok level. Must be s sensor issue.

the flyin' scotsman

Is there Dot 5 in there now?

Is the tansmission manual?

Use Dot 4 brake fluid from a new sealed container.

Mrmerlin

I would suggest to use the ATE super blue racing fluid,
dont use any syn fluids as they may cause the seals to swell thus requiring a new master cylinder and caliper rebuild

longleader

+1

the flyin' scotsman

From the owners manual:

GlenL

Go get a pint of DOT3 or DOT4 at the local store. Why a pint? So it'll get used up and not be on the shelf for a long time getting old and watery. You only need DOT 4 if you actually drive the car on the track.

dr bob

DOT 4 is the same cost as DOT 3 so no reason not to use it.

Modern brake fluids are already synthetic, so no worries with using a compatible synthetic fluid. As others remind us, Do Not Use silicone fluids.

I used a synthetic Valvoline fluid at my last change, replacing the Castrol LMA that wasn't available locally/conveniently at the time. It's working fine. Stan's recommendation for ATE is great, especially when you are replacing all the fluid. It's blue instead of clear so you can see when new has replaced old. Roger sells it if you can't find it locally.

the flyin' scotsman

ATE is blue or gold so flush depending on whats in there now.

Dot 4 is Porsche recommendation for newer cars.

depami

For normal street driving why would one want ATE blue or gold over Dot 4?

Is there a performance advantage for normal street use or is it just that the color is handy for flushing if you alternate between the two?

dr bob

ATE is rated DOT4. Difference is color and maybe a higher wet boiling point vs the DOT4 rating. Since the hydraulic mc is ATE branded, there's no question about compatibility.

Probably the best dot4 fluid is the stuff you flush with every year or maybe two. Dry anything trumps the best fluid wet every time.

The OP's question was about what to top up with. Any DOT4 fluid is fine. When you get around to flush and replace, the more esoteric fluids might make sense depending on how you drive and maintain your car. For most of us that are street-only drivers, fancy fluid buys nothing but bragging rights I guess.

Randy V

I just completed a system flush and new Blue refill.

The braking feel and performance is extraordinary.

dr bob

I rest my case.

Gary Knox

ATE blue is extremely good for higher temp braking (track, etc.) It does have a downside, in that it is my hygroscopic, thus absorbs water faster than the Castrol or other similar fluids. As it absorbs water, the boiling point drops of course, and steel rusting becomes more of a problem. I purge ATE blue in the '89 track car NO LESS than once a year, and usually twice.

Gary--

the flyin' scotsman

while its true its easy to determine when the fresh fluid is present when using a different colour not all brands offer anything but the standard gold.

What would be helpful if one could 'dye' gold fluid to any colour without affecting the brake fluid properties.