Dave has it right there. Some Harrisons have problems and some don't. It may be rear pad differences. Most Falcons with that MC had drum brakes. Some folk are upgrading the front brakes and that might accentuate the problem. Not sure.

Thats what it is folks, say it 3 times slow or quick, just say it and know what it is.. "Pressure differential Switch !! Pressure differential Switch !! Pressure differential Switch !! " Thats what she said.
If you blow a hose or run short of fluid and the MC starts to pump air because your fluid evaporated for whatever reason, including worn pads and caliper pistons well extended and hence consuming all available fluid reservoir , too small reservoir, maybe a blown primary seal in the MC or whatever then the piston which is centred by a (relatively) light spring and balanced hydraulic force is displaced and a ramp activates the simple switch to mostly go to earth. This sparks up the dash lamp indicating brake fail or it is possible to have the handbrake and brake fail lamp connected so that either/ or will activate the lamp and having the ISO pic with brackets and exclamation mark (!) or similar is OK, but check with your approving person or locality.

Many people never think of the poor cheap thingy switch, but it is there to serve us and save. As Boxy pointed out if the light is on the switch is tripped. LOOSEN the switch a few turns to reduce the pressure on the spring activated pin then pump the brakes a few times to centre it (the balance piston).
The one rule that is rarely followed is,, When you bleed your brakes LOOSEN the switch a few turns, this will ensure the activation pin is not broken as the piston moves around and it will basically auto reset when the system is bled and all is balanced.

The switch is a very simple circuit that is easily forgotten about but even more easily managed. It's a PITA looking for a new switch if you break it on a sunday arvo.

Not lecturing, I just like to support the little bits that work for us.

Gary.
(spokesperson for often forgotten and neglected and mistreated parts)

Stumpy
I was scratching my head over the switch as well, on all the time, until i realised the 2 poles are connected inside.
The solution........the switch earths through the mastercylinder, so you only need a single wire returning to the dash lamp, ie 12v ign supply to the lamp, then the earth running from other side of lamp to either pole on the switch. When the plunger is pushed in by the piston (as per Gary's eloquent description) it completes the circuit and on goes the lamp.

If the switch is a single wire one then yes it earths through the switch and master cylinder, But I thought (again going by old mechanics memory) the XF had 2 wires?

I know the 540 I am building has 2 wires.

And a big Hoorah for the little parts that make a big difference :)

Lionels XF scans
 

Yes the XF switch has 2 wires. Safer to have a remote earth where it suits you. I think XY onwards (in PBR anyway) has the 2 pole switch.

Apologies for the delay, but here are the scanned pages that Lionel sent in PDF for your viewing pleasure. 1.66 mb, I didn't rotate them straight as it caused text loss.
XF master cylinder destructions

two wires
 

My original circuit looked like Boxheads diagram.
So I cut an old one in half to see they are in fact joined inside, hence the query "Where did the second wire go to on the Ford"
http://www.clubcobra.com/forums/pict...d=5058&thumb=1

Point taken, good question.
Off to the gregory electrical schematics.

The second wire goes to your ignition switch so the brake light comes on at start up to check the bulb.

Hmmm, I see. well looks like I lead you up a wrong path.

The wiring in mine was done by my sparky mate (guy who owns 540 I am building) so I did not actually wire mine, but the lights do come on with the ignition.
I did do engine harness.

Many are also wired in parallel with the handbrake warning light.

Thanks LT65. That makes sense, as it checks the wiring up to the switch as well.:)

Job well done. Not normally keen on red leather but yours looks great, like the silver too.
I see Your comments on rear view morrors, do you have to have them or can you use rear facing active cameras as my new G6ETurbo has an auto dimming rear view mirror but have wondered if you could use rear camera as rea view.

Would you be able to see the screen in a car with no roof? Ive been in a car with a double din and you cant see the screen with the sunroof open.

G6
rules state you need a centre and drivers side mirror, a question for an engineer.......can't say id go for an lcd screen in the middle of my cobra dash though

Just to clarify, your ignition switch is positive, so connecting the "second" wire to the ignition switch would result in positive going to both sides of the brake fail lamp when starting - lamp wouldn't light. Also, if your brake fail switch was earthed (eg, low fluid pressure), you might have 12v pos from the ignition shorting through the brake fail switch - yikes.

So, the "second" wire on the XF brake fail plug is just another potential earth for the brake fail lamp. To get it to work, your ignition start circuit should activate a relay which causes the second wire to connect to earth (chassis), causing the light to go on while the starter cranks. Remember the other side of the lamp always has 12v pos.

The tricky part (to me anyway) is that the ignition starter relay activates an earth circuit to the lamp, much like the brake fail switch, whereas we normally use relays to control positive circuits. Eg, headlights, fans, etc.

Hope this makes sense.

the fiddly bits are the hardest to sort out
most old school auto elects have no ieda and use old poor parts see it many times
just had to work with a pre made fusebox wiring kit from rocket industries which customer supplied and had to mod the back of it so one fuse can be used for something else and pulled on wire gently and it came out
so beware as u all know most sheit comes from china and quality and tightness of crimps are pisspoor


anyway quickest and easiest way is to connect alternator wire thru diodes to turn on the handbrake or low brake fluid light
this way when alt turn on the charge light it turns on the others
if u do have barina brake fluid resevior sensors they have 3 wires
an earth and a brake light wire and an alternator wire
they have diodes in the cap which does it for u

u should see the show car i just finished today

its called a goliath
old 40s or 50s pommy car
2 door station wagon
has a matching 4 link custom trailor
trailor and car is on air bags
has old school v8
should be at mequires this years
nice green colour

The tricky part (to me anyway) is that the ignition starter relay activates an earth circuit to the lamp, much like the brake fail switch, whereas we normally use relays to control positive circuits. Eg, headlights, fans, etc.

Hope this makes sense.[/quote]

Yep a relay will do it or you could have a Falcon ignition switch that fits nicely on the Laser steering column that has an auxillary contact that you can earth to do the job. Works for me.

Greg - how did the softer rear pads go? I picked up a new XF Falcon master cylinder (wagon - orange reservoir: 3:1) cheaply yesterday. May fit it at some stage and see if that makes any further difference.

http://www.clubcobra.com/photopost/d.../medium/MC.jpg

For those still interested in this thread and in relation to an earlier comment by Greg re modifying the existing PBR XF system to increase rear wheel braking, I did find this:

Proportioning valves

Some cars with rear drum brakes will have a proportioning valve; these are often set in the range of 150 to 250 psi. As rear discs are not self energizing like drums they need higher pressure settings, typically 500 to 800 psi. Possibly the rear requirements of your car might need a pressure so high that no off the shelf valve comes in this setting. You might be able to delete the nylon spool from inside the existing prop valve so that it no longer works at all, this means that in an emergency you might lock the rear brakes if you press hard enough, but usually well after the front brakes have locked. The acceptability of this will depend on how the rears perform in emergency situations, which means a careful road test.