12 replies to this topic

[newenglandmini]

How do I test this master cylinder mounted brake pressure switch for serviceability?

 

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[KernowCooper]

The plunger on the end moves in, so connect a multimeter to the 2 terminals on the other plug end and put the meter on ohms, touch the 2 meter probes together and the meter should read 0, place the meter probes onto the 2 terminals and press the centre pin, 0 shows a circuit is being made in the switch.

[newenglandmini]

Thank you Dave.  That is what I did prior to making my post, and did not get continuity when I depressed the plunger.  I wanted to confirm that my test was the correct one.  I'll place an order for a new switch.

[Spider]

Actually, those switches work the other way. They have a circuit with the pluger OUT and break the circuit when the pluger is pressed IN.

 

<EDIT: I don't think direct replacement switches are available any more. The new ones (black in colour) work by making a circuit back through earth>

Edited by Moke Spider, 06 February 2015 - 02:36 AM.

[newenglandmini]

How does the switch work to indicate a brake failure?

[Dan]

There is a shuttle valve inside the master cylinder that is positioned between the two circuits. A pressure difference between the two will cause the shuttle to move one way or the other and when it does a ridge on the shuttle trips the switch. You should be aware that BL dropped spares support for these master cylinders decades ago and ideally they should be replaced where possible with a yellow tag front to rear split master. If you have a diagonal split system I don't think it can be changed but if it's front to rear you should consider it. The replacement was originally the subject of a service bulletin as the shuttle valve setup is not entirely safe, if the valve fails the two circuits can be linked.

If you have the type where the switch is in a separate valve block by itself then it's exactly the same, the PDWA is a shuttle between the two circuits and if it fails they can end up linked.

Edited by Dan, 08 February 2015 - 05:51 PM.

[Spider]

There is a shuttle valve inside the master cylinder that is positioned between the two circuits. A pressure difference between the two will cause the shuttle to move one way or the other and when it does a ridge on the shuttle trips the switch. You should be aware that BL dropped spares support for these master cylinders decades ago and ideally they should be replaced where possible with a yellow tag front to rear split master. If you have a diagonal split system I don't think it can be changed but if it's front to rear you should consider it. The replacement was originally the subject of a service bulletin as the shuttle valve setup is not entirely safe, if the valve fails the two circuits can be linked.

If you have the type where the switch is in a separate valve block by itself then it's exactly the same, the PDWA is a shuttle between the two circuits and if it fails they can end up linked.

 

 

Yes. The thing is, they don't work. Dan has explained quite well how they should work, but they don't. Also, I've always found that a failure in one circuit always renders the other inoperative in any case. Pointless really.

 

When these MC's are fitted, I remove that switch and plug the hole (only so it looks 'right', it shouldn't leak from there) and fit a cap from a Yellow Tag MC, the one with the float switch. At least they do work in giving indication of low fluid (and a possible leak).

[newenglandmini]

My master cylinder was unserviceable and I learned that it is NLA.  I consequently purchased a GMC 227 from minispares.  I haven't opened the box, but I trust that it will have the upgraded switch.  I posted my original query because I'm in the process of going through every bit of my disassembled car and I had forgotten that I have a new cylinder in stock.

 

Thanks for your help and for the interesting history of this part.

[Spider]

Yes it should have. That's the Yellow Tag Cylinder Dan was referring to. There is a plumbing change you also need to do to fit it.

[Dan]

And a wiring change. If you look in our FAQ section you will find the original BL service bulletin paperwork dealing with this change which details the plumbing and wiring work needed. The original BL pipe work to go with the M/C is also still available.

[newenglandmini]

The minispares website helped me obtain the necessary plumbing when I purchased the MC, so I think I'm sorted on that issue.

 

Dan, I searched the brake FAQ section, but I did not find the BL S/B.  Would you kindly give me a specific reference to assist me in locating it.

[Dan]

I can't find it either.

It has gone.

In fact quite a few things have gone.

This bears investigation.

[newenglandmini]

I opened the master cylinder box.  A clear set of installation instructions (including the wiring modification) is included with the cylinder.

 

Thanks again for your assistance.