MGB Register Forum

The differential piston is a bar with two recesses on .If the two circuits are not in balance and/or you get a leak in one of the circuits then the bar slides until the button tip of the switch uunderneath the master cylinder body releases into one of the recesses & then makes the warning light circuit complete. Hence the need to unscrew the switch 31/2 turns when bleeding the brakes to stop the button tip jamming/ remaining in one of the recessess.John Twist has a useful videko on this on utube.
Cheers, Charles

Thanks for the info Charles.

Thanks guys

"You can't bleed the brakes properly until the electrics are working because there's no way of knowing whether the differential piston has been decentralized without the warning light working."
Are you saying this couldnt be checked with a multimeter or continuity tester across the pressure warning switch ?

You could try and see if there's a circuit between the 2 contacts on the switch using a multimeter set to circuit continuity. Make sure the switch is screwed fully in first.
If there is a circuit then it's essential you bleed the brakes per the manual, starting by slackening the pressure warning switch 3 1/2 turns.

The switch under the master cylinder body that i referred to earlier is a simple push button switch. When the button is depressed because the differential piston (bar) is in the correct position (i.e. not where the plip of the button is released into one of the indents are).
The switch has a unique socket & plug attachment to the loom. You can get a new switch as it is used for MGBs & some Triumphs but the plug/socket end of the loom is (to my knowledge) not available as a separate item. (I managed to source a used one cut off from an old loom from Andy Jennings if i recall correctly as mine was all corroded). With the plug / socket fitting disconnected from the switch it would not be easy to check as and when the "circuit" is made or broken using a meter unless some one else is operating the foot break. (the pins within the bottom of the switch are close together making the use of crocodile clips impossible. You could cut the wires away from the plug / socket fitting leaving it connected to the switch & then just attach crocodile clips of the meter to the wire ends & then you could watch the meter as you use the foot brake. (Afterwards you would then have to reconnect the cut wires with bullet & sockets.(I am not recommending you do this as once the loom is in place through to the cockpit & brake warning light you could do the test then)!
Cheers,Charles

Charles, I had a look at the connector to the pressure switch and I see what you mean about the connector. I suppose some sort of connector could be made but I agree it would be simpler to connect the electrics first then apply the brakes and see what the warning light does.

The plastic pressure switch on my car had broken off flush with the bottom of the master cylinder before I got the car. After consultation on here, I just ignored it. It doesn't appear to do anything important. Mind you, I haven't had to bleed the brakes yet!

Dave,

The fact that it has broken will not in itself stop the brakes from working. It's function as i am sure you are aware is to make the driver aware if there is any loss in brake fluid in either the front or rear circuits. I personally check before i set off (as my car is not a daily driver) that the brake fluid reservoir is full as a double check. I know that many owners of these later cars have blanked the threaded hole off , being concerned that at some stage there might be a leak through this hole if not filled by the plastic switch body - some taking the view that by the time the warning light comes on (if the switch works & is properly set up) that loss of braking is more or less immediate particularly if it is the front circuit where the problem occurs.I hasten to add that the course of action taken above is not something i would recommend!

Cheers,Charles

Dave, that's what happened to my switch when I started to loosen it prior to brake bleeding. I did extract the remains.
If you have to bleed the brakes, use a pressure or vacuum bleeding system. So long the brake pedal isn't moved during the process the differential piston shouldn't move.
However, you'll have no way of checking if it had.

As indicated earlier, the actual brake failure switch is readily available - part number AAU2454 , but the plug/socket attachment to the wires from the loom is not detailed as a separate available item.
Cheers,Charles