Brake reservoir wouldn't hold pressure or fit.
I've been mulling this over. The liquid is likely to slosh around, left to right as you turn and front to rear as you brake / accelerate. Floats can be tempermental. The expansion tank is under pressure and the liquid gets pumped about, albeit ideally not through the expansion tank but who knows when coolant is low?!
A neighbours old VW has a low coolant alarm. It is built into the side of the reservoir and contains a high and low wire, a bit like the 2 screws trick. Now, I don't know what the difference in resistance between air and water / coolant mix is but I am guessing some sort of tuning is required? Either way, first thing is to have a play with a light / sounder, some wires and a battery in the garage and test the principle. After that I'll probably need a relay and maybe a potentiometer.
Moving on, it seems to me the head is the most important thing so bolting a sensor onto that seems like a good idea. That brings me back to the Engine Guard idea. In the end I'll probably end up doing both in some guise of another but I just need to have a think and a play first.
In the meantime the car did about 30 miles today and lost no visible coolant. I'm pretty sure it is losing only a little from where the coolant pipes go on the heater matrix tails. The clips I put on are the worm gear type and I think the heating / cooling cycle loosens them up. I am hoping (constant tension) clips like these with a spring might be an improvement:
We have discounts and trade terms if you want to buy Supra Constant Tension Clamp - 304SS - Dia=29-31mm from HCL.
hcl-clamping.co.uk
StepLess® Screw Clamps are designed to provide a durable sealing for environments with strong thermal cycling. Self-tensioning StepLess® Screw Clamps are high performing clamps used in cooling systems. They are designed to withstand thermal cycling and increasing pressure – typical conditions in...
hcl-clamping.co.uk