NovacaineFix
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From my years as an auto-tech, Dot 3, 4, 5.1 are not compatible with Dot 5 simply because of the base fluid is different. The advantage of 5 over the others is the fact that it is silicone based and does not absorb water or moisture, which is the main cause of brake system component failure. That sludge you talk about is the result of consolidated moisture that has attacked and corroded metal parts within the system.On the 3 vs. 5 debate, you guys touched on a curiosity of mine, I would like to hear more input.
So basically, when ever you have an open container of Dot 3, 4 or 5.1, it is absorbing moisture from the air, you can't do anything about it except only open the container on super dry days. That moisture will tend to collect in the calipers and wheel cylinders where the heat from braking starts to boil that moisture. Being that water is heavier than brake fluid, it collects in the furthest bottoms of your brake system, corroding everything away until it is flushed out.
Whenever I did a brake job for a customer, even if they didn't pay for a full flush, I would push that fluid out of the calipers and cylinders as part of the brake service and replenish any fluid that was lost.
As for DOT 5, as per the interwebs search, it has a much higher boiling rate than the others, about 500° over DOT 3's 400° and DOT 4 is rated somewhere close to 450°, DOT 5.1 is rated for almost 520°, but it is still a glycol based fluid like 3 & 4. Since 5 does not absorb water aka moisture, it cannot promote corrosion within the system. The only drawback is it sometimes creates small air bubbles as it absorbs air. This is why cars and trucks with ABS systems do not and cannot use DOT 5, the fast moving actuators that pump the brakes for you instead of locking up would churn up the air inside the fluid causing a erratic braking scenario.
As for the seals, if you are converting from one to the other, I believe it would be best to change out the seals to eliminate any possibility of cross contamination within the system. I have never had to do a fluid conversion myself, so I would refer to anyone else who has be the expert for that. Not sure if you can just clean all the seals if they are in good shape and change fluids.
IMO, DOT is far more expensive, but worth the extra cost, especially for a vehicle that may not be a daily driver and not get as much maintenance as other vehicles.
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