View Full Version : TR2/3/3A DOT5 brake fluid

05-07-2019, 12:25 AM
I saw a note from Rock Auto wherein Dot 5 must never used with ABS brakes. The rapid stroking makes foam. Only bad news. Another note, my brake lite switch failed and if Silicone was the problem, I opened the switch to find the diaphragm intact and functional. It is to press a brass disc against the back ends of the 2 terminals. It looked like it closed one but not both terminals. one about "dirty". One suggestion is an air bubble formed at switch and not enough pressure could be made. I spent $10 for a new switch and now all's well. Le T.

05-07-2019, 12:58 AM
An air bubble shouldn't keep the switch from working; even with the bubble, the pressure is the same throughout the system.

But many of us have had similar problems with the TR3 style switch. The "dirt" on the contacts appears to be burned brake fluid. Silicone has a reputation for "creeping", and it seems like somehow it manages to creep into the cavity where the contacts are.

Not everyone has the problem, so it probably varies from switch to switch; but I've had several failures of both original switches and replacements from several different sources (including one sold by FLAPS for an old Ford pickup truck). I eventually got frustrated and converted to a mechanically operated switch instead. Since I didn't like having it exposed under the hood on my previous TR3A, I put it under the dash on the TR3.


The switch shown did malfunction after a few years, mostly I think because I had a MC leak and some of the DOT 5 ran down the pedal and found it's way into the switch. But I replaced that with a TR6 type switch (which also fits Stags) from TRF, and it has been working fine ever since.

I've also heard rumors that there is a Harley Davidson motorcycle switch that will fit in place of the original TR3 switch, and stand up to DOT 5. But I've never investigated any further than that.

05-07-2019, 07:10 AM
The hydraulic switch problem is not unique to TRs. It seems to happen to a large number of people who convert their cars to DOT-5.

The Harley Davidson switch is mentioned a lot as a solid replacement. However, I have also seen a number of threads that warn that only the OEM Harley switch is "good" and that their aftermarket switches also fail.

Like Randall, on a couple of my cars I have made the change to a pedal actuated switch.

05-07-2019, 10:59 AM
I converted 2 cars to DOT5. On one the switch began leaking within a month and got replaced. It never leaked again for 5 years until I sold the car. The second car...even older!...is on it's second year after the conversion with no issues with the original 1955 switch. I personally think this is like converting your engine to synthetic and having the seals all drip. If the switch was marginal to begin with, the silicone will kill it. If the switch is fully intact, it is not an issue.