Brake Line Pressure

[handi_andi]

Hi All
I had to replace my brake master cylinder a while back and as a result have lost the brake warning light system. It has struck my mind that it ought to be possible to incorporate a simple pressure switch into the brake line system to operate the brake warning light in conjunction with the brake light switch. Firstly, has anyone else built such a system? Secondly, does anyone know what sort of pressure the brake lines operate at so that I can source a suitable pressure switch please?
Thanks
Andrew

[ppnelan]

Fom the www: "Typical brake line pressures during a stop range from less than 800 psi under "normal" conditions, to as much as 2,000 psi in a maximum effort."

Matthew

[CBUEB1771]

Andrew,
What is it that you wish to indicate? The brake warning lamp fitted to Elans indicates low fluid level in the master cylinder reservoir. A pressure switch will not provide this information. An indication of low line pressure in and of itself is not valuable unless it is coupled with another input indicating what the line pressure should be.

[handi_andi]

I wish to have some indication that all is not well wiuth the brake system. Hence I was thinking about taking the input of brake pressure and the brake light switch being on as the two inputs. As to my mind if the brake lights are on then there shoudl be brake pressure. Hence if the system is turned on by the brake light circuit and then inverts the input from the brake pressure switch, if there is no brake pressure but the brake lights a on then the warning light would come on. That way it would not only indicate low brake fluid, but also a loss of pressure caused by a failure of the master cylinder seals for instance.
Andrew

[rgh0]

If no pressure in the brake system when brake lights on because the pedal is pressed you dont need a warning light as you get instant feedback via the pedal going to the floor and that "oh sh*t" expression coming out of your mouth.

cheers
Rohan

[john.p.clegg]

Andi
What I have done is take the electricity from the brake light switch and feed it to the Hand brake lamp.....you need to fit a diode......to stop non live supply causing problems connecting to live supply......and a resistor...to cut down on the brightness.....now you get two messages from the lamp...hi-bright handbrake and low bright brakelights.

John

P.S.
when I talk about live/non-live I'm talking about the ignition switch positions.

[mikealdren]

Andy,
Don't add a pressure switch or gauge, it's just another potential point of failure and you don't want that in the brake system.
best wishes,
Mike

[CBUEB1771]

Don't add a pressure switch or gauge, it's just another potential point of failure and you don't want that in the brake system.

Mike makes a good point about adding points of failure. However I did just run across an approach to adding a pressure switch with minimal additional fittings. Pegasus (www.pegasusautoracing.com) list banjo bolts with an integral pressure switch for braking applications. These are available in inch and metric thread and for single and double banjos. I am sure these can also be found in the UK.

[handi_andi]

Thanks Russ. I have come scross something similar in the UK since putting up my original post, as well as a straight screw in brake line pressure switch. I will, unfortunately in some ways, be increasing the number of failure points in the brake system anyway as I want to fit a bleed screw just after the brake servo anyway to allow me to properly bleed that part of the system. My plan was to install a three way union post servo, with the original line from the servo in one hole, a new line to the servo in the other hole and a bleed screw in the third. If I was to increase this to a four way union then I would have somewhere to install a screw in pressure switch. My own experience of hydraulic systems failing is that they are more likely to go at flexible hoses or the seals in pistons than on solid pip plumbing. Hence to me the risk intriduced by the extra solid pipe plumbing in low whilst the benefits gained are high.
Cheers
Andrew

[mikealdren]

Andy,

Sounds a reasonable solution.

Personally, I would not expect to need a bleed nipple at this point. I find that the narrow bore of brake pipe carries any bubbles easily to the calipers and I've not had trouble bleeding +2 brakes.

To return to the failure aspect, my concern is the sensor itself. Pressure sensors for engine oil pressure do fail and leak - it's irritating and messy but not really dangerous. Look carefully at the resilience of whatever you use.

Also bear in mind that brake components are highly regulated. On a more modern car, you would probably fail the MOT with the mods you are suggesting, I'm not sure with an Elan.

Mike