Thanks that explains the difference between the different types of sensor.
As to the safety.
Im fairly experienced with vehicle mechanics and braking systems.
Ive rebuilt more than I can count on bikes as well as cars.
I have qualifications as well as commercial experience.
The pressures or lack of pressure with the vacuum is fairly low.
On a normally aspirated petrol engine the vacuum is taken from the carb manifold.
So relying on induction process, the sucking in of the fuel air mixture in the carb.
Lots more chance of failure if you take into account the heat and other possible failures in that area.
In this case its a deisel, not so much induction vacuum, so it uses a pump run mechanically by a push rod on a cam I think on the crank shaft. Maybe another lay shaft.
All of the pipe fixtures are press fit and the pipes take a torturous path along the bulkhead with several joins and T's in the system to change the ID of the pipe for the other ancillaries such as the EGR system and one way valves. Plenty of places to tap into the pipe and keep it secure against the bulkhead. Ive already replaced a number of the vacuum hoses that looked like they were on there way out tracing the last problem.
This is an assisted brake system the backup in case of failure is to push on the brake pedal harder.
As I had to do on the motorway the other day when it failed towing a heavy braked trailer full of all our camping gear with 2 adults and 4 teanagers in the car. I wouldnt have wanted to try an emergency stop as I had to practically stand on the brakes as it was. Towing and loading weights were well within the limits for a landrover discovery.
The accepted test for the vacuum pump is to put your finger over the open pipe to see if its sucking.
As to the servo yes you can use a rule of thumb on the change of pressure on the pedal.
I cant find any published figures for the system yet. but im looking.
As anyone whos run a car or bike has found you tend to compensate when things wear or degrade until it stops working. Someone else driving the car may notice its not as good as it could be and you then take notice.
My problem was that the brake performance faded over time with eventual full failure in the vacuum servo and then a week later, after replacement of the servo, the pump finally gave up. In hindsite the old pump didnt suck as well as the new pump but I had nothing to compare it with when I replaced the vaccum servo.
It felt ok to me at the time but there was a compareable difference to the new pump.
The ABS system the car has can can pick up some of the failures or degredation in the braking system but the vacuum assist has no failure warning or monitoring. So I wanted to tap in a sensor and let it monitor over time and let me know of any long term degredation that would warn me of possible failure.
Fairly easy to monitor and report something like this with the Arduino. But Im sure Ill expand it like the other guys did with the car turbo projects.
Gordon