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MG MGA - Is my brake switch bad? HELP!

Hi folks, last week I started a thread concerning the brake lights not going off on my 1600 after a pressure bleed.

After some investigation I discovered that my right rear wheel cylinder was sticking. I subsequently removed both rear wheel cylinders, had them honed by a specialist, and fitted new seals etc as well as a new flexible brake line and re-bled the system (Dot 4).

The brakes now feel superb. they pull up strongly and straight and the pedal is firm. The brake do not drag (I can spin each wheel easily by hand). ... however my brake lights are still not going off!

I've checked for residual pressure by unscrewing both rear cylinder and front caliper bleed screws with no luck. I've even partially unscrewed the the brake line from the master cylinder into the junction and again the brake lights stay on and there is minimal loss of fluid.I should also state that there is sufficient clearance between the brake push rod and the piston in the master cylinder.

If I turn the ignition off, wait 5 minutes and then turn the ignition back on the brake lights do not illuminate.

Is it possible that there is enough residual pressure in the line to keep the brake switch on but not bind the brakes?

Or is my brake switch faulty? I have a nice looking original as a spare but don't want to go to the hassle of replacing it (car is RHD) if it's not the switch.

Any suggestions? I have a national meeting in less than 2 weeks and I'm running out of patience!

Cheers

Steven
S HILL

My bet is that you have a bad switch. The best way to confirm that is to replace the switch and see if that cures the problem. With the ignition off, disconnect the wires from the switch, remove the switch and install the new switch and reattach the leads and try it out - if the brake lights go our when you release the brakes, you have found the problem. Don't worry about having to bleed the brakes or about spilling any brake fluid while changing the switch - just put a rag of some kind under the brake light switch to catch any fluid that might ooze out, but that doesn't normally happen. Cheers - Dave
DW DuBois

Thanks Dave. I suspect you are right. I've run a continuity test across the two switch terminals with the ignition on and the brake lights not illuminated and I'm getting a reading. There must be an internal short in the switch.

Regards

Steven
S HILL

Test the switch with an ohmmeter.

It should be open with no pedal pressure and closed with pedal pressure.
A Bowie

Thanks fellas, all sorted now. Getting the old switch off without scratching any paint was a job and a half. It must go down as one of the more frustrating jobs I've ever done on the A. Unfortunately my nice original spare was a non goer so I've had to resort to using one new aftermarket switches. Reading through the archives I guess I can expect to replace the new switch soon!

Regards

Steven
S HILL

Why not use DW DuBois' suggestion for a spark supressor on the switch (it's on his website)? The capacitor worked out at 10p and the relay with the diode built in was £3.50 This is thought to dramatically extend switch life.

I have to say that with such difficult access to the switch position and the risk of damaging the connected hydraulics I opted out and fitted a mechanical switch.
Graeme Williams

I refuse to jury rig non-standard parts on the car just to allow use of junk replacement parts. Just buy a good brake light pressure switch and be done with it.

The reason we have so many faulty parts on the market is because people keep buying the junk and then refuse to send it back. If the bad parts are not returned, the supplier assumes they are perfectly good parts and will continue to sell the same junk forever. If many people send the faulty parts back for refund, the suppliers would be losing money and would stop selling those parts, and the problem would not exist.
Barney Gaylord

This thread was discussed between 14/03/2016 and 18/03/2016

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