In line brake pressure switch failure

[andrewh]

Having covered some 3200 miles since restoration the brake pressure switch has decided not to work. Not surprised really. Usual story of inferior quality parts no doubt.

Has anyone experienced this and found a solution or are the parts better than they were when I restored my 3.8 a few years back?

thanks for any input.

EDIT: I meant to ask as well, presumably one can change the switch without a complete bleed of the brake system?? That really will pig me off!

[Ruurd Schut]

The hydraulic switch is mounted in the piping on the cross member for the front brakes, if you need to bleed the system (which I don't expect) it is just the front brakes. I changed my switch without bleeding, bought at SNG Barratt in the Netherlands.

[mgcjag]

Hi Andrew....a while ago "Cactusman" was testing the "new improved sngb brake switch".....its on tne forum somewhere....he did say it was good. ....fluid will come out if you change the switch......I think it would be sensible to bleed the system.....if your on standard brake fluid it should be replaced every couple of years anyway.....you could consider the " upgrade" non hydraulic electrical brake switch some have fitted attatched to the brake pedal...Steve

[andrewh]

The hydraulic switch is mounted in the piping on the cross member for the front brakes, if you need to bleed the system (which I don't expect) it is just the front brakes. I changed my switch without bleeding, bought at SNG Barratt in the Netherlands.

Yes I know where it is. I fitted it ! The SNG switch is the one that’s failed ! Thanks

[abowie]

Try buying one for a 1960's Volkswagen Beetle. These are what I'm using on all my cars.

Note that the thread is M10, not 3/8" but that none of mine leak.

[andrewh]

thanks Andrew, I will take a look at those.

[Heuer]

Time to fit a mechanical switch!

[cactusman]

I'd contact SNGB....the ones they now sell are their own brand and seem better than the intermotor rubbish. Julian Barratt gave me an early sample of their improved one and thus far (4 years I think) no problems. I did fit a relay so the switch operates the relay and the relay switches on the bulbs....considerably reduces the current switched by the hydraulic switch....

[abowie]

. I did fit a relay so the switch operates the relay and the relay switches on the bulbs....considerably reduces the current switched by the hydraulic switch....

The switches I've pulled apart have a big thick copper disc and two pretty robust contacts in them. I don't think they fail because of excessive current. With a 21W brake light on each side the total current draw is only 3.5A.

Inside the switch the brake fluid is isolated from the copper disc by a rubber diaphragm. Under normal circumstances when you brake the rubber diaphragm bulges up and pushes the disc up with it, bridging the contacts.

What happens in failure is that fluid leaks past the diaphragm into the switch area. So there is now fluid on both sides of the diaphragm. Then, because the fluid in the switching area is incompressible, when you put the brakes on the increased pressure can't push the rubber diaphragm up and make the disc bridge the 2 contacts.

[Heuer]

Andrew is correct. The repro ones sold today are extremely light duty and don't hold up to the 3 amps drawn by the brake lights for very long before the contacts burn again - something to do with the contacts not being silver plated - if your original works keep it. If it ever does fail then you have problems. This is the problem:





Here is a dismantled E-Type switch and you can see how the diaphragm operates a pin which presses on a metal 'biscuit' which in turn is held away from the contacts by a spring.




Applying the brake causes fluid to move the diaphragm, pin and biscuit to make a circuit. Unfortunately the contacts arc with the 3 amp load as you can see from the two examples below. The arcing causes a build up of carbon and the switch quickly fails:


From what I can gather the original Lucas switches had heavier gold or silver plated contacts which prevented the arcing but for cost reasons they now use non precious metal. Keeping your foot on the brake when stopped in traffic makes things worse - a practice that was not common back in the day. If you are wedded to the idea of retaining the original hydraulic switch you need to install a simple relay to remove the load.

A mechanical switch solves the problem provided you use a quality switch.

[cactusman]

I would agree....the original ones are good for 5 amps....the intermotor reproduction ones are not. I have dismantled more than one repro. Some fail due to hydraulic leaks preventing closure. Others have failed as the cheap contacts have burned...I have not yet pulled an SNGB one apart....never the less there is no harm in adding a relay...a modern relay will switch 30 amps which is way more than you need