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Read this morning that Subaru is recalling over a million vehicles for brake light failure.

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Haven't really studied brake switches to know their construction and what kind of barrier separates the fluid from the electrical section -- suspect some kind of rubber -- so this theory is probably more in Owen_Dyneto's field of expertise. Seems logical to me that if Subaru is having the problem with their cars from vapors off a cleaning solution used at some point getting thru a barrier then the relatively fast failures of switches used in our Packards that have been converted to silicone fluid could be laid to a constant emission of vapors coming from the silicone fluid doing the same thing.

The article did not say what Subaru is going to do in correcting the problem but would imagine they have a switch replacement of some kind that is impervious to the vapor.

The Subaru brake light switch is mechanically operated by the brake pedal and not connected to the hydraulic system.

I halfway suspected it was mechanical as very few use hydraulic switches anymore but still if vapors can affect the contacts that easily I wouldn't rule out the same in the hydraulic type switches.

No idea on the size of silicone vapor molecules suspected of causing the problem but if small enough they could pass thru a barrier membrane that is still capable of not admitting any liquid.

A good example of small molecule issues was found when Freon was banned. Because the R134 molecule is smaller than the R12 it was found that some of the old rubber hose formulations could not completely contain the new refrigerant hence a new hose formulation and different liner designated "barrier hose" became the standard. The old hoses will still mostly work in original R12 systems that are converted to R134 because the oil molecules that circulate with the refrigerant have penetrated into the original hose pores and act as a barrier to the smaller R134 molecules. If a hose had to be replaced and then the new refrigerant was used to fill the system the issue became obvious because the refrigerant started leaking and system was low in a fairly short period of time. A lot of mechanics spent a lot of man hours looking for leaks in fittings or changing components when the cars came back before it became common knowledge the hose was where the problem really was found and new barrier hoses became readily available.

I'm not sure that synthetic brake fluid is necessarily silicone as the container just says synthetic.

I am speaking of the DOT 5 fluid which is silicone based.

As you may remember a few years ago many recommendations were to drain the DOT 3 and 4 fluid which is hydroscopic. The older fluid tends to corrode or deteriorate brake systems if ignored because the fluid attracts moisture over time which leads to the issues. Theory was if replaced with the 5 fluid which did not attract moisture it was thought there could be a longer term no maintenance and storage option for cars which did not see much use.

Of course all did not go as planned and because of the incompatibility of the fluids, there were lots of issues and even some failures with systems that just had the old fluid drained and then replaced with 5 and no flushing or cleaning step in between. Because of that I don't believe there is as much thought going into replacing the 3 and 4 fluids now.

Properly done it seemed and probably still is a viable option but the reports began coming in that the brake switches suddenly failed shortly after the fluid changes. Many just chalked it up to the old switches finally meeting their expiration date but I believe failures are still somewhat of an issue with those running DOT 5 even with new switches. Some with failures have converted to mechanical switches and others have just replaced the hydraulic switches as needed.

I have both DOT 4 and DOT 5 synthetic in my various cars and never any problem but most are '60s cars with mechanical brake light switches. Other than replacing all hoses and rubber components, then thorough pressure flushing and bleeding, I've done anything special when switching over.

You said the magic words needed with the proper method for that particular fluid change-- changing rubber and hoses and pressure flush. Those steps apparently were not done by a lot of people and from what I remember reading, those with issues -- aside from having brake systems in questionable condition -- just drained one fluid and filled back up with the other with not even a minimal flush between the two. You wouldn't necessarily even need to change the rubber and hoses if in good condition and a thorough flush is done before the final fill but I guess many skipped even that step.