Brake Light Switch and DOT 5

[Joseph P. Indusi]

I just replaced the brake light switch on my 53 Special that uses DOT 5 silicone brake fluid. I only got 2 years and less than 2,000 miles on the last one I replaced. It seems the 2 year old one would only cause the brake lights to come on if I pressed real hard on the brake pedal. With the new one it seems to work normally.

Does anyone know why this happens with DOT 5 and is there a way to prolong the switch life?

Joe, BCA 33493

[old-tank]

Joe

It is the crappy switches. I have to replace the switch nearly every 2 years on my cars with glycol fluid...once in 12 years on the one with silicone fluid. On my long distance driver I have a mechanical switch since the hydraulic switch would be very hard to replace on the road.

Willie

[ply33]

...On my long distance driver I have a mechanical switch since the hydraulic switch would be very hard to replace on the road.

Willie

I guess that depends on the car. On mine the switch is in an easy to access location and I've successfully swapped it just by pulling the wires, unscrewing the old, screwing in the new and putting the wires back on. I probably got a little air in the system but the pedal is still high and firm. Since I can get that switch at any decent auto supply store on short notice it is no big deal to repair it on the road.

[Jolly_John]

Hi, Joe. Your switch life problem (when using Dot 5) is a subject that has been discussed periodically on many old car forums. The silicone brake fluid seems to creep and wick into the contact area of the brake light switch, acting much like an insulator.

If periodically replacing this switch really bugs you, about the only thing to do is go to make an underdash bracket, and go to a pedal-activated brake light switch. I have seen some guys even leave the wires to the master cylinder-mounted brake light switch in place as dummies, while connecting the pedal-activated switch for "real". John

[Roadmaster75]

I have these pressure switches in all my old cars. Two of them are DOT5.

The DOT5 cars get about 2 years ..... maybe 3 if I get lucky.

The DOT3 car (my 58 Roadie) goes about 3 .... My local NAPA gets them for me.

I buy 3 or 4 at a time and keep them handy.

I have learned to switch them REAL FAST while asking some unsuspecting neighbor or girlfriend to "Hold your finger right here"......! Never had to bleed one yet

mike

[Joseph P. Indusi]

I think old-tank is correct. The silicone must get past the rubber cover over the copper cup that presses against the contacts when pressure is supplied. I better rubber seal at the edges would prolong the life of the switch.