Although you now have to have 2 separate brake circuits for safety & SVA there is no way of knowing that a circuit has failed until the inevitable control difficulties under heavy braking.
I'm thinking of making a brake comparator circuit using two brake pressure switches in the front & rear lines & a couple of relays. A light on the dash will come on when the brake pedal is pressed & only one pressure switch activated. It's known as an Exclusive OR (XOR) circuit.
Has anyone else done this? Had any problems?

Sounds a very easy and sensible idea!!! Piss of pi$$ to add before completion of the car, a little bit more complicated afterwards (you'ld have to break the brake lines) but an hour or two should do the job....

Not done it, but the light will flicker/flash as the switches come on at slightly different times which might be irritating. Also I'm not sure how much additional warning it will give.

How are you going to make an xor out of relays?

Making an XOR out of relays is easy......
Two relays, +ve into NO, -ve into NC, common into one side of a lamp. Coil powered up by the signal from the brake pressure switch..
Normally (ie no brake) lamp will have GND on both terminals, on braking you'll get 12v on both terminals, one on but no the other, lamp goes on...

Jobs a good'un...
True- a bit of flickering is to expected, as previously mentioned brake switches work on quite a high pressure and on light brake pressure you might get one coming on before the other....

Cunning (didn't even think about bulbs being non-polarised, been using LED's for too long )

Drew it up to make it easier for my poor old brain to understand, though in reality it would probably use switched earths for the pressure switches.


Description

Yes that looks correct. I realise the lamp might flicker a bit but it shouldn't be too distracting. I might try a capacitor across the bulb & see if that reduces it.

mhh

but when you have a bias bar you would almost loose both circuits right???

also if you put a cap on the bulb it wont lit up (in parallel)

the cap will need to charge first but then the 2nd relais is there and it discharges true the bulb

there is a formula to calc how big it needs to be to give a certain amount of delay

Tks

No matter what you do if either hydraulic circuit fails your are in deep doodah without any warning except for a sinking brake pedal which is instantly obvious.

If you really want to fit a differential pressure warning there is a special switch available that was used on some cars with Girling diagonal split systems, but for obvious reason it won't work on a balance bar setup.

[Edited on 22/3/07 by britishtrident]

the simplest answer is to build the braking system properly and have it inspected if you are not confident it has been built right.

99.9999999999999999% of all cars on the road seem to manage without this complicated stuff you are planning, why do you feel you are unique in that you require this?

If a circuit fails, then the fluid will get out. When the fluid gets out the fluid level switch activates, and the warning light comes on.

I understand what you are adding, but wonder whether you are just increasing your chance of a failure (more joints, more leaks, etc.).

I guess in the situation where you lose a circuit and before the fluid is depleted this will give you some indication.

Its your build, have fun with it.

Matt

Not meaning to rain on your parade or anything, but sounds like a waste of time to me.

What we call "creeping featurism" in the software development business.

You become mired in the "hey wouldn't it be cool if it did this" stage, the project gets overcomplicated and crappy / never finished.

There's wisdom in KISS (Keep It Simple, Stupid)