Saw the low brake fluid warning yesterday whilst stationary on a hill. It self-cleared but will check the level. There seems to be some confusion regarding the use of dot 3 or dot 4 fluid - I thought they were compatible but thought I'd check before messing up. Car is an M3P. Cheers.
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Dot 3 or Dot 4 brake fluid?
- Thread starter spon88
- Start date
From the service manual, DOT 3:
Both 3 and 4 are glycol ether based, with 4 having a higher boiling point. Unless you're using it on track the boiling point is largely irrelevant.
Both 3 and 4 are fine to mix if you already had a bottle of DOT 4 available.
Um. The brake system in modern care are, like, rather extremely sealed. The fluid doesn't evaporate, for example. Most cars I've had over the decades have never had a low brake fluid alarm.
In the one or two cases where there has been such an alarm, it's because there's been a leak. In one case on a VW a ruggedized rubber hose had been somewhat misaligned so, with the steering wheel hard left, a tire rubbed against a hose. In another car, rust eventually got to a metal brake line.
So, even if the alarm (a) went away on its own or (b) is gone because you've filled the reservoir, right about now would be a good time for a serious brake inspection. Not the least of which would be to have the car up on a lift or something while somebody pressed hard on the brake pedal while somebody else looked for leakage. Any red-colored puddles or spots under where the car's kept parked? Loose joint? Road damage? Dunno, but this is the kind of thing one doesn't want to fool with.
In the one or two cases where there has been such an alarm, it's because there's been a leak. In one case on a VW a ruggedized rubber hose had been somewhat misaligned so, with the steering wheel hard left, a tire rubbed against a hose. In another car, rust eventually got to a metal brake line.
So, even if the alarm (a) went away on its own or (b) is gone because you've filled the reservoir, right about now would be a good time for a serious brake inspection. Not the least of which would be to have the car up on a lift or something while somebody pressed hard on the brake pedal while somebody else looked for leakage. Any red-colored puddles or spots under where the car's kept parked? Loose joint? Road damage? Dunno, but this is the kind of thing one doesn't want to fool with.
WannabeOwner
Well-Known Member
You won't want to see this then!
Spoiler: He should have replaced the DOT 3 with DOT 4, and not topped up with DOT 4. (follow-up video explains, if you are interested). The reason he hasn't got a back window is from an earlier video when he drove the car under about 6 feet of water ... his channel has some entertaining stuff in it!
Interesting! I was aware that dot 5 + is not compatible but most publications suggest 3 and 4 are - but don't use them on track.
I will check thanks. No sign of leaks however.
electricar
Member
The brake fluid level goes down as the pads wear and that will eventually cause the low fluid warning to come on.
Um. Brake cylinders, like those found in the disk brakes on all four corners, have nifty little shiny pistons that push on the pads that make the brakes brake. Each of those pistons have bitty rubber o-rings, sometimes two per piston, that keep brake fluid in and not out. A leak anywhere around there would likely hit a spinning wheel and get deposited who-knows-where, and maybe only under high-pressure events. Or when the brakes are really hot. So feeling around back there and looking for excessive grime, yeah, you want to do that.
I’m just a lowly shade-tree mechanic, and not that good of one in any case. My advice is worth what you’ve been paying for it.
General rule of thumb has been that brakes are, in general, the one item where a dealership’s repair department does a really good job. Mainly because after-failures with death and all, juries are filled with drivers who know perfectly well how brakes work, have no sense of humor whatsoever about a bad brake job, and the dealership is a large, brick and mortar operation with easily findable principals and deep pockets who a jury with the right mindset wouldn’t mind backrupting.
As a result, dealerships might put trainees on regular maintenance items like oil changes, but they tend to be more careful about the people who work on brakes.
The above isn’t 100%, of course, there’s always a bunch of idiots somewhere, but there you are.
WannabeOwner
Well-Known Member
Indeedie. That was the issue in the video - race-brake-pads and thus a lot more heat in the brake fluid than normal (although the supposition was also that the MS Plaid ought to have better "brake infrastructure" as standard). He wasn't properly hammering it (no helmet or anything else as he was just doing some "short bursts", but he did have racing pads fitted and a mix of DOTs ...) but he still came a cropper. But he does that quite often in his videos!
The online Service Manual for the Model 3 says DOT 4. This particular BASF product is hard to find, but the specs available are the same as the DOT 4 "Low Viscosity" fluids such as Pentosin DOT 4 LV or Liqui Moly SL6 DOT 4 Brake Fluid.
Here is what it says as of today:
| Brake Fluid | DOT 4 BASF Hydraulan 404 (part number 1012041-10-C) is the recommended brake fluid. If DOT 4 BASF Hydraulan 404 is not available, DOT 3 Dow 372LB (part number 1060099-00-A) can be used alternatively. Note: The brake fluid filler cap is marked 'DOT 3'. This is the minimum requirement and does not state to only use DOT 3 brake fluid. CAUTION: Only use new fluid from sealed containers. |
