I agree the hydraulic switches operate, or should operate, at low pressure but they are certainly not instant – that is one of the reasons why car manufacturers replaced them with a mechanical switch years ago. And they are not very reliable, especially new ones. When you brake, whether hard and fast or not, first the mechanical clearances in the pedal linkage are taken up, then the piston in the master cylinder moves, fluid is pushed into the slave cylinders and calipers, the pads and shoes move and pressure increases – then the hydraulic switch operates and the brake lights come on. Obviously it takes much longer to describe what happens but you’d be surprised how slow it is and even a van moves a relatively long way in a fraction of a second and the guy tailgating you could have hit you before he even knows you are braking. A mechanical switch operates as soon as your foot hits the pedal, before the clearances in the linkage are taken up and before the brakes operate.
I’m simply stating facts; at 50mph you’re covering more than 73 feet a second. I’m not saying there is anything wrong with the hydraulic switches they work, or should work, just fine but they can’t operate with no pressure so the brakes are applied before the brake lights warn anyone following that you are braking. Many car manufacturers thought that the delay in brake light operation was important and an improvement could be made by changing to a switch operated by the brake pedal, even the Mini and MGB were modified.
How can the brakes be operating without pressure Switch operates when there is pressure in the brake system Brakes operate when there is pressure in the brake system I have seen lots of old pedal switches that have not been adjusted properly so are more likely to be a problem of delayed action I would say manufactures changed to pedal switches as they are cheaper.
