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MG MGB Technical - Better Brake Light Switch - Early or Late?

I was all set to replace the hydraulic brake light switch on my '67 with the mechanical switch from a later MGB when I found the thread about adding a relay to the early switch.

My complaint on the hydraulic switch sold by Moss Motors is that it takes too much pressure to light the brake lights. I want drivers following me to be aware that I am about to apply the brakes, not that I applied the brakes several seconds before they plowed into the rear of my car.

So the question is, if I can use either type of switch, which would be better, assuming I already have the parts needed to convert to the later switch? Is a relay advised for the later switch, or just the earlier? Thanks. -G.
Glenn G

Glen - neither switch as supplied by Moss is worth a tinker's damn. Just about all the replacement switches available at a reasonable price are junk. The hydraulic switches, when new (and those protected with a relay/arc suppression circuit) will light the brake lights at a very light pedal pressure. The failure mode on these switches is that it takes progressively more an more pedal pressure to get the brake lights to come on. The mechanical switches without a relay/arc suppression circuit will just suddenly fail outright. The other draw back of the mechanical switches is that they can be misadjusted such that the brake light will be on all the time, or at the other extreme will leave the pedal slightly depressed and cause constant pressure on the master cylinder, which results in the brakes being constantly applied.

The choices are 1) go with either the standard replacement switch, either hydraulic or mechanical and a relay/arc suppression circuit (see the article Brake Light Relay in the Other Tech Articles section of my web site at: or get a heavy duty hydraulic switch (P/N SW 32) from Ron Francis Wiring at: Cheers - Dave
David DuBois

Virtually all modern carts have a mechanical switch, and failures seem to be quite rare. At least none of my cars has failed, and they all have 100,000+ miles. Many of these are screw in types (since they are typically adjustable for position of action), which may or may not fit in the existing threaded boss (on some cars), but any of them could easily be fitted under the dash on a simple bracket. Pay attention to the switch position as Dave mentions - it is not a pedal stop, and it should actuate the brake lights at very small pedal stroke = before brakes actually work.

Fletcher R Millmore

Pointless adding a relay to a hydraulic switch, which is what you seem to be asking, if the problem is that the hydraulic switch takes a lot of pressure to operate. The benefit of the later mechanical switch is that it will light the brake lights almost as soon as the pedal starts moving, which will always be sooner than a hydraulic pressure switch can light them. This is a safety benefit due to the earlier warning warning to following drivers (presuming they are looking in your general direction!) much as LED brake lights do compared to incandescent.

As such the mechanical switch is clearly better, but it does have to be adjusted correctly, whereas the hydraulic just screws in till it stops and that's that.
PaulH Solihull

Oh yes, *either* of current stock are likely to need a relay!
PaulH Solihull

Thanks for the help, guys. I will go with a standard mechanical switch and relay until forced to try something else. -G.
Glenn G

You might try this link below if the switch in your car is the same as used in an MGA. It's for Ron Francis Wiring Co. They have a low pressure hydraulic brake light switch. I have been using it for well over 2 years and it has been very reliable. No failures. Good quality and turns on the brake lights just as you touch the brake pedal. It's model SW-32. Here's the link:

Bob Ravich

This thread was discussed between 26/07/2010 and 28/07/2010

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