Re: D.O.T. 3-4 vs. D.O.T. 5 for a '56 Patrician

Posted by Tim Cole On 2015/9/18 15:56:01

I will say this about the hydraulic brake switch problem.

The ones I had problems with were replacements. I scrounged up an old one once (35 120) and it still worked fine. But as I recall, it too became intermittent, thus my suspicions about modern DOT 3.

The system is not a good one because the switch is load bearing. I think the best way of using an existing switch is to use a 6 volt relay. That will reduce arcing damage inside the switch and convert it to a sensor.

I don't think flushing a DOT 5 system is necessary. The owner's manual for my modern car says nothing about flushing the brakes either. There are thousands of Toyota Corollas running around with 300,000 miles on them with the original fluid. The bigger problem is rust on the surfaces that are exposed to air because the shoes aren't worn yet. When DOT 5 is used a corrosion retarding film is created when the cylinders retract. With DOT 3 a rust zone is created.

Another aspect of this issue is that, if water intrusion was such a big deal, modern master cylinders would not be vented they would have an expansion chamber. You would see reservoir caps with a spring loaded diaphragm to handle compensation and replenishment. There's a great aftermarket gimmick.

So I would prefer DOT 5 on drum brakes hands down. Especially because during assembly the exposed surfaces are coated with a corrosion retarding film. DOT 3 creates a rust promoting film.

Some suppliers I used said they didn't want DOT 5 used with their parts. Okay. And some of those parts failed anyway so they were just playing the blame game.