11 replies to this topic

[SKIDDERWOLF]

My black tag master cylinder has decided to let go and coat the interior of my car in brake fluid - Lovely!

I have 7.5" discs fitted so will take the opportunity to change to the yellow tag master cylinder.

Apart from the master cylinder I understand I need new brake lines between the M/C and the pdwa valve due to the yellow tag having metric fittings and the Pdwa valve being imperial.

Looking on minispares I have found this kit BAU5654 would this be the one I need in addition to purchasing the master cylinder? 

 

 

 

 

[mbolt998]

You may also need to adjust the wiring if your old MC had a pressure differential light. I think the yellow tag has a low fluid warning light instead (at least mine does) and the logic is different. I managed to fix it with some cunning soldering where the wires go into the switch/light but there is a part available that you just somehow plug in somewhere. I'll find the post where I tried to explain the soldering if you are interested :)

 

Check also which circuit the top and bottom cylinders of the MC go to. I have 4x drums and the bottom of the MC goes to the rear circuit. It's probably the same with discs, but not sure. With the old MCs my understanding is it didn't make any difference as the bores were the same, so it was quite common for the lower part of the MC to go to the fronts.

 

Those look like the right brake pipes. I just made up my own. They're metric into the pressure thing on the bulkhead (if you have that kind) and also metric at the MC.

[SKIDDERWOLF]

I noticed the sensor thing will need to be changed but I can do that ok, if you have a link to the post that would be interesting to read thanks.
I will take a look into making up my own brake lines. If they are both metric I can get some parts ordered in.

[Spider]

The Threads in the Yellow Tag are M10 and M12. Those in the FAM7821 are M10.

 

The wiring changes are straight forward - both have switches that make in a faulted condition, you'll just need to change the crimp connectors.

 

You may also benefit from changing the Rear Wheel Cylinders. It's likely you have 5/8" ones there now, you maybe able to go to 11/16" types. This will balance off the change in rear pressures that you'll get with this master.

[SKIDDERWOLF]

Thanks Spider, yes I have 5/8" on currently.

[Ethel]

That's a M12x1 thread, academic if you buy a conversion kit. Note the bottom, M12, outlet is for the rear circuit.

 

You can't use a yellow tag GMC227 with a diagonal split system (just in case somebody else reads this topic in the future).

[mbolt998]

The Threads in the Yellow Tag are M10 and M12. Those in the FAM7821 are M10.

 

The wiring changes are straight forward - both have switches that make in a faulted condition, you'll just need to change the crimp connectors.

 

 

My wiring changes weren't straightforward. I thought I had written this up in more detail on here, but I don't think I did.

 

In both cases there are two wires going from the bulkhead to the MC. On the old MC they terminated in a black plug which used to plug into the side of the MC, which was a pressure differential switch. So first I cut off the plug, and attached two spade lugs, but it didn't work.

 

The way the pressure switch is wired is documented in the wiring diagram.

 

 wiring.jpg   67.58K   3 downloads

 

 

159 is the switch/light on the dash, and 160 is the brake pressure differential switch in the MC. P is purple, and live. B is black and runs to earth.

 

The pressure differential switch has two wires going into it, in that plug. The genius of the design is that the socket they are plugged into connects them together when there is no fault, and to earth when there is.

 

When you press the button on the dash, it tries to turn the light on by earthing the bulb, but via the plug and the socket. This means that if the plug has fallen out of the MC, the light won't come on when you press the switch, which is perfect, because you know something is wrong with your safety system that you need to investigate.

 

If there is an actual brake pressure differential fault, the bulb is earthed and the light comes on that way.

 

The new MC is as you describe: the contacts are connected together if the fluid has fallen out. But that won't make the light come on with the old wiring unless you are also pressing the button on the dash. That's not how it's supposed to work of course: you're supposed to press the button to test the light and the wiring, and if it comes on without you pressing the button, it means you're going to die.

 

So here's how I fixed it:

 

 mods.jpg   118.93K   2 downloads

 

The green line shows the path of current from +12V to earth (as drawn it's passing through both switches-- the one in the MC cap and the one in the dash button). But 160 is now just a switch, which closes when there is a fault. The modifications are shown in pink. We cut one wire where I've put the X and add two others. If you follow the paths you will see that now the light should come on if either 160 is closed or you press the button on the dash.

 

These changes can be rigged up with the wires behind the switch (I used solder plus electrical tape) and all stuffed back behind the switch panel. Or, as I later learned, you can buy a part which I think does this, or something similar.

 

https://www.theminif...-4#entry3746013

 

[Spider]

On the late cars, that's possibly how it was (and what you found), The Fail Switch on the Master Cylinder would have been Black on yours and the two terminals in the 'switch' have a shorting bar connecting them together. The earlier types, up to about 84-ish were White and were a Normally Open (Close on Fail) Switch with two contacts.

With these late types, the conversion is still easy and straightforward. The Black/White Wires are commoned together and connected to one terminal and the other terminal to Earth on the new cap

[mbolt998]

On the late cars, that's possibly how it was (and what you found), The Fail Switch on the Master Cylinder would have been Black on yours and the two terminals in the 'switch' have a shorting bar connecting them together. The earlier types, up to about 84-ish were White and were a Normally Open (Close on Fail) Switch with two contacts.

With these late types, the conversion is still easy and straightforward. The Black/White Wires are commoned together and connected to one terminal and the other terminal to Earth on the new cap

 

That's interesting! My car is a 1979 Mini 1000. The plug going into the MC was certainly black. Can't remember what was on the MC (I lost that original MC 30y ago or so when the garage changed it).

[mbolt998]

With these late types, the conversion is still easy and straightforward. The Black/White Wires are commoned together and connected to one terminal and the other terminal to Earth on the new cap

That sounds like it ought to work looking at the wiring diagram. My solution was perhaps more complicated :) Whatever the case after rigging it up OP should test for the desired behaviour: light should come on when pressing the switch and everything OK and also if you remove the cap from the MC to simulate no fluid.

[Spider]

 

With these late types, the conversion is still easy and straightforward. The Black/White Wires are commoned together and connected to one terminal and the other terminal to Earth on the new cap

That sounds like it ought to work looking at the wiring diagram. My solution was perhaps more complicated :) Whatever the case after rigging it up OP should test for the desired behaviour: light should come on when pressing the switch and everything OK and also if you remove the cap from the MC to simulate no fluid.

 

 

Given that the Black 'Switch' is not really a true switch but a contact, that makes an Earth Contact via the Shuttle in the (internal) PDWA Valve that's all that should be needed.

[SKIDDERWOLF]

Thanks everyone for the advice regarding reconnecting the switch. I've not had chance to investigate any further but from what I've read it sound straightforward to convert the wiring to suit the yellow tag MC.