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MG Midget and Sprite Technical - Brake Failure Test Switch-Explained

The archive make is difficult to find the answer to this question so I am creating YET another thread about the brake pressure warning test/lamp.

I have a new switch. It has 4 prongs but they are not labeled. The OLD switch was labled 1-5. I made the mistake of just removing wires without marking where they went. One had a smaller spade. I cut the terminal off to replace with new so I have no idea which one had the smaller spade. I have one purple, two black with white tracers, and a ground. I have also burned 6 fuses trying to figure it out. The wiring diagram is useless.

Does any one know what color wire corresponds to the number terminal?

Please explain the circuit. Does the bulb light when there is a loss of pressure, without pressing the test button? Does the bulb illuminate if you press the button, even if there is not pressure failure? I read one archive that said the button only test the bulb.

So confused.

JAM Meador

JAM. I assume that they are using the same system that was used on the MGB. If not, please disregard. The light will illuminate when there is a pressure differential between the two sections of the master cylinder. This may indicate that there is a problem, but not always. If the light comes on and you feel a change in brake operation, pull over to the side and call for a tow truck. If you do not feel a change in the function of the brake system, drive carefully to a good repair shop and have the system inspected for problems. I have had two "false positives"--i.e. the light coming on when there was not problem with the system, and two brake failures, both of which were evident as the pedal was applied and before the warning light come on. Not a great fan of the system myself.

Yes, there may be a "press to test" feature associated with the light. On my 68 BGT, there is such a feature. On the later Bs the light comes on with the ignition switch and stays on for a few seconds before shutting off when the engine starts. Both systems are designed to allow you to know if you have a good bulb with the second system being a requirement because our government found most of us too stupid to check the function of the warning light periodically.

Les Bengtson

Hey Jam,

Look on the 5 way union where all the brake lines come togather (Moss #48 & 49) its on the passanger side wheel well area in the engine bay, there will be a plastic piece that screws into that 5 way brass body and has 2 wires coming out of it...Im guessing those 2 wires are the black with the white tracers..the purple wire is for insrument lights, so its probably the purple that powers the little lite bulb some how

remember that is a rocker switch above the steering wheel that has the red light in it.

And on that brass section where the 2 wire plastic part that screws into the 5 way brass union....its has a spring loaded switch that is built into the bottom half of that 2 wire plastic piece,,,if you unscrew it a small amount it makes a pop and resets its self....Then agian, maybe thats was just my old hag pag brake system ....all I know is I dont miss it, Im still in love my tilton engineering brake system

prop...Vaniity, My red dash light switch is just for looks...Prop

the button is not to test the light but to reset the shuttle valve rod inside the brake block.

When you bleed the brakes for example, and press on the brake pedal hard and a bleeder screw on a wheel cylinder is not tight or is leaking, then the light will come on. This example simulates a failure in either the rear or the front brake systems...( a leak, broken brake pipe, leaking wheel cyl, etc.

Inside the brake block the shuttle vave is centered....a leak will send the valve to one end or the other and turn the light on. You will still have 1/2 of your system that is working.

The brake block are rebuildable but there are only a few outfits that are capable. H.T.H.
M. Who

>>> the button is not to test the light but to reset the shuttle valve rod inside the brake block. <<<


Not so, I'm afraid. All the switch on the dash does is to close the circuit to the brake warning bulb so you can see if the bulb's good or not. And yes, on the later midgets the light illuminates while cranking and then goes out, and the "button" doesn't push in at all... it's just a filler plate with a red lens in it.

Gryf Ketcherside

I hate to say it, but you are slightly mistaken. The SWITCH is there to test the light, but the light IS there to report on the state of the shuttle.

OTOH, M. World Health Organization seems to be a trifle confused on the chicken/egg issue. The switch does absolutely nothing to reset the shuttle, the light merely reports that the shuttle is not centered (the metal tip of the insulated probe is contacting the shuttle and grounding out. No, this does not mean it needs rebuilt, merely re-centered. Page 158 of my Haynes manual, Chapter 9 (Braking System), Section 4 (Brake Hydraulic System - Bleeding), under Dual Circuit System describes the process. Yes, the process works.

As M.WHO states, the shuttle system allows the system to continue working on half the circuits in the event of a leak. At least in theory. The light is intended to let you know that this has happened and you need to fix your brakes.

The switch merely grounds a wire which runs out to the shuttle assembly and back through the other wire to validate the facts that the switch is still good, the bulb is still good, the wire is still connected, there is still some juice in your battery, and if the shuttle is not centered, then at least it isn't grounding out the sensor for some reason. Not exactly a reliable indicator of the state of the world.
David "my light works" Lieb
David Lieb

This thread reminds me of my search to find the answer in the archive. Lots of different and contrary explanations. I found the attached by doing a google search.

JAM Meador

>>> OTOH, M. World Health Organization seems to be a trifle confused on the chicken/egg issue. The switch does absolutely nothing to reset the shuttle <<<

Right, that's what I said.


Gryf Ketcherside

Pack sand !
M. Who

Sounds suspiciously like the kind of response I have come to expect from those WHO hide behind a pseudonym.
David "yes, that's my real name" Lieb
David Lieb

This thread was discussed between 24/09/2009 and 04/10/2009

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