[malb]

For the brake warning,
1. There is an amount of leeway for the piston before it starts to blanket the feeder tubes and reduce brake efficiency. Generally if you see the piston groove through the switch mounting hole, brake lines should all be open and working.
2. There is no leeway with the switch, if the groove in not centred in the switch mount hole, the switch will indicate.

Piston offset could well be a result of a brake bleed some time ago, rather than current leakage anywhere. Unless fluid is changed regularly, there is a chance that moisture will start surface corrosion in the chambers, and the piston will jam if moved over the surface corrosion. It appears that there are no seals involved with the piston, it is a firm metal to metal fit in the cylinder. As such detection system is non recoverable. I have had to replace one on a 240 and tried to reco the original as a spare.

Despite opening both ends, the piston could not be removed to clean up the surface of the cylinder, so the unit was scrapped. Have looked at a number in scrapyards over the years and all similar. Basically, unit will indicate fault once after the onset of surface corrosion, then can't be reset as the piston has moved and jammed in the corrosion buildup.

Re temp guage, circuit is basically 12V from ignition into 3 terminal regulator, approx 5 V out from memory, to guage then to ground via sender in head. The sender is a temperature dependant resistor, and hence changes the current flowing through the meter. Meter is normally thermally activated, rather than coil and permanent magnet, to give a slow (time averaged) response.
Regulator output also shared with fuel guage making both independant of battery voltage. Shorting the sender lead to ground for a short period would not show on the guage, as it has a long averaging period, (1 minute +), and long term shorting is not advisable as it can cook the regulator and guage. From memory, I suspect that the working range of the sender is from about 50 ohm to 150 ohms.