| First, let me say that there’s no such thing as inferior brake fluid. All brake fluids sold today meet one of the DOT specifications, so no matter which one you use, it’ll work fine. Even if you use fluid from a unsealed container, the worst thing that can happen is that the fluid absorbs moisture and will boil if it gets to 300ºF or so. The same thing happens if you don't change your brake fluid once a year or so. The absorbed moisture in the fluid will, however, eventually corrode expensive brake bits over time.
In fact you could put straight water in there and it would work, it would just boil at a lower temperature (212ºF.)
DOT 3 and DOT 4 fluids are compatible with DOT 5.1 - but are not compatible with silicone-based DOT 5. DOT 5 brake fluid is purposely dyed purple to warn against accidental mixing, so you should be able to tell right away if someone added any purple fluid. Another way you can tell is that the DOT 5 fluid will coagulate in the reservoir when it’s added to any of the others.
One concern is that DOT 5 does not absorb water, so any moisture in the hydraulic system will "puddle" in one place. This can cause localized corrosion in the hydraulics.
Also, Brembo products are said to use natural-rubber base seals that are incompatible with DOT 5 silicone-based brake fluids, and there were reports of DOT 5 causing premature failure of rubber brake parts with early DOT 5 formulations. This was found to be due to improper addition of swelling agents and reportedly has been fixed in current formulations.
However, nothing discussed above will lead to brake lock-up. The worst thing that can happen is that you start to have piston retraction problems so the pads drag continuously on the rotor. If the pads drag, the calipers and rotor can get pretty hot as you’d expect. But again, there’s no way that this will result in a lock-up condition.
So when they say they think that it’s a brake fluid or a master cylinder problem, they’re really saying that they just don’t know.
Two possibilities come to mind:
The first is that they failed to assemble the brake mechanicals properly during a recent service, they saw evidence of this, and decided to pay for the repairs (without admitting fault and incurring only limited liability.) I once had a situation where the owner of a Ducati dealership forgot to tighten down the four pinch bolt holding down my front axle, so believe me mistakes happen. If something was loose when it left the dealer, a mechanical interference problem in a left hander (not necessarily right away) is one way that the front wheel would abruptly stop turning.
The other possibility is just bad luck. A piece of metal in the road could get kicked-up and lodge between a rotating and stationary brake (or suspension) part. A small bolt caught in one of the rotor’s cooling holes, for example, will stop the wheel quickly enough to cause you to loose traction, shear off, and leave you wondering what happened.
Since you don’t know what really happened, my suggestion to you is this: Let it pass. There’ll come a time when you may need help from this dealer again. In my case, I went back to the owner (remember he was the one who made the mistake), took him aside and explained his mistake and (suggested) the wisdom in having the shop’s work double-checked when there were obvious safety-related and liability issues. The next time my bike was serviced, I was treated like royalty.  |