Them's the brakes

December 16, 2011 11:28 AM

So I'm doing dome work on a 99 Volvo XC and, not paying attention, filled my brake reservoir with DOT3 fluid instead of DOT4. If I don't drain it and replace it with DOT4 am I taking a terrible risk?

Also, I'm using one of those little hand vacuum pumps and don't seem to be drawing anything through the system with it. Is this indiciative of another problem (something with the ABS hydraulics) or is it just that I'm using a barely adequate tool and would be much happier with something like this?

Or should I just go back to having my wife press the brake pedal while I work things at the wheels?

posted by Kid Charlemagne to Travel & Transportation (4 answers total)

Best answer: No, you are not taking a terrible risk using DOT3 fluid, but DOT3 brake fluid has a lower boiling point and more readily absorbs moisture, so it is inferior to DOT4. In milder climates this is less of an issue, but in more extreme climates, or if you are towing heavy loads or routinely descending steep mountain roads, this could be an issue.

A quick explanation: Polyglycol Based DOT3 - Color - Amber | One of the most common finds on the market and have a specific for cars designed for the late 80s period. DOT3 also absorbs moisture the quickest and has the lowest boiling point and should only strictly be for road use.

Polyglycol Based DOT4 - Color - Amber | The benchmark specifications by the majority of car manufacturers. DOT4 is designed to absorb moisture slower than DOT3 and has a higher boiling point. This is both suitable for road use and track work. Using racing specific DOT4 refined brake fluids will increase higher temperature tolerance particularly under hard braking conditions.

Silicone Based DOT5 - Color - Purple | Least common and also the most different from both grades above. DOT5 are primarily designed for cars not equipped with ABS units (Unless otherwise manufacturer specific) and this should not be used unless you're absolutely sure of what you're doing. DOT5 does not absorb moisture and holds the highest boiling point. This brake fluid is not compatible with either categories above and should never be mixed due to it's chemical properties. (Via)

I can't shed any light on your hand pump, but I have the Motive Power Bleeder that you link to and it's fucking amazing! I can now do brake jobs all by myself without having to ask my long-suffering wife to help, something we both appreciate. With the power bleeder, I can bleed all four wheel cylinders plus the load sensing proportioning valve on my Tacoma in ~10-15 minutes, solo. I'll never willingly go back to the old method.
posted by mosk at 11:42 AM on December 16, 2011 [3 favorites]

bleeding the brakes is pretty easy, especially if you have a pedal pusher. if it were me, I'd just buy a new can of different brake fluid and push all the old stuff out of the system. if you get a fluid with a different color to it, it's really easy to bleed at each corner until the stuff with the new color shows up.

you CAN mix them without any noticeable effects. DOT3 has a very low resistance to boiling and moisture, and as a result can even contribute to fade under certain street driving conditions. and if you mix them, you're gonna essentially downgrade the DOT4. if you're doing the work, you might as well do it right and purge all the 3 out. brake fluid is like $10 a bottle, all its really costing you is time.

sure you can buy a power bleeder, but unless you bleed brakes a lot, why waste the $75. i've bled brakes on almost every car I've owned and still don't think it's worth it.
posted by ninjew at 11:43 AM on December 16, 2011

As said above no worries on mixing the two. If you are concerned grab a turkey baster and suck the mix from the master cylinder and refill with DOT4 and then bleed.
posted by alfanut at 1:55 PM on December 16, 2011

Response by poster: Thanks all - One of the forums I've gotten a lot of good information from had a number of people who had me convinced that everything I loved would die if I went ahead and flushed with DOT3 but there was no clear explanation as to why. I expect I'm going to have to do my rear pads and rotors next summer, so I'll just flush the whole mess then.
posted by Kid Charlemagne at 3:35 PM on December 16, 2011