Bare "plug" near brake master cylinder

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Ian Fozzard
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Re: Bare "plug" near brake master cylinder

Post by Ian Fozzard » Fri Apr 24, 2020 8:58 am

Thanks Charles,

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Charles Farran
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Re: Bare "plug" near brake master cylinder

Post by Charles Farran » Fri Apr 24, 2020 9:07 am

The second barrel has a bar with Vs in it . If the pressure is incorrect the bar moves and the switch buttonp pops out to fill the void. (The switch is off when the button is pressed in, hence when it pops out the electrical circuit is created & the warning light comes on).The difficulty is setting the bar in the right place when bleeding the brakes. The switch has to be unscrewed by a few turns when bleeding to make sure the switch button doesn't sit in one of the Vs & prevent the bar from moving to it's middle & off position.
Cheers, Charles
1980 Roadster

Ian Fozzard
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Re: Bare "plug" near brake master cylinder

Post by Ian Fozzard » Fri Apr 24, 2020 9:12 am

Even better! I am enlightened (at least on that issue😁)

Thanks

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Michael Barclay
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Re: Bare "plug" near brake master cylinder

Post by Michael Barclay » Fri Apr 24, 2020 9:25 am

Hi Guys
Great job Paul. I guess the next question is why it was disconnected. Looking at the diagram the brake fail switch is effectively wired in parallel with hand brake switch/dash light. My suspicion is the brake fail switch failed and the previous owner disconnected it to prevent the dash brake light being permanently lit.
I would suggest disconnecting the brake fail switch as it looks in poor condition and checking if the handbrake switch and dash light work correctly. If this is OK then check the operation of the brake fail switch with a multimeter switched to Ohm or continuity setting. Pressing the plunger should see the switch contacts open and close. The other possibility is the diode in the live feed from the starter motor solenoid (White /red wire) to provide a failsafe live feed from the ignition.(the normal feed is via the green wire from the fuse box but the fuse could fail hence the failsafe ) From memory this is a black plastic block with a male and female lucar connector on either end. With 12 V on the cathode side (Triangle) with the diode connected in circuit you should get approx. 11 V on the anode side (Line)
As Ian mentioned it would be interesting to see what has been substituted for the switch in the brake line.
Hope this helps.
Best regards
Mike Barclay

Dave Wheatley
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Re: Bare "plug" near brake master cylinder

Post by Dave Wheatley » Fri Apr 24, 2020 10:05 am

Just had a good look at it.

First, the threaded part of the switch has broken off the main part, either accidently knocked, or bust when trying to remove from underneath the master cylinder. Attempted removal possibly because of switch failure - the 2 pins on the switch have corroded away - not sure if this would result in permanent warning light, or no light at all.

The threaded part is still in the cylinder, and the central hole is still open. No visible fluid leakage. The Haynes manual 1962 to 1980 page 157 shows the detail of the pressure differential unit.

The handbrake warning light works as it should.

As has been mentioned, this switch/valve doesn't appear to serve any usefull purpose, so I am inclined to either leave things as they are, or to try and remove the plastic threaded portion from the cylinder and plug the hole with a suitable bolt and sealing washer, or run a suitabe stainless self tapper and washer into the central hole in the broken off plastic threaded part of the switch still in the cylinder as a backup seal..

Thanks to all for for the replies to this esoteric problem.
1978 MGB GT
1931 Austin 7 RM
1955 Standard 10
1972 Ford Cortina
No modern!

Ian Fozzard
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Re: Bare "plug" near brake master cylinder

Post by Ian Fozzard » Fri Apr 24, 2020 10:20 am

I only have single line brakes on my 72 BGT, but looking on the positive side of this system on the later MGBs, it seems to me that if one of the dual circuits did leak to some degree, it may not be noticed by the driver, but would be picked up by the switch?
I think it's the case that if one circuit fails completely you would still have the other circuit to provide some (unbalanced) braking, provided you hadn't run out of brake fluid!!
So probably this system could serve some useful purpose in some circumstances?

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Charles Farran
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Re: Bare "plug" near brake master cylinder

Post by Charles Farran » Fri Apr 24, 2020 12:37 pm

Dave,

If the pins have corroded then they would probably fail to complete the circuit whether or not the switch plunger is in the "out" position (i.e. on) or in the "in" position which is off. When the circuit is made the brake warning light comes on. (I had these circumstances - you only need to touch the two wires together that feed the plunger switch , with the ignition on, but the handbrake off to see if the bulb itself works & that the circuit to the plunger switch connector is Ok (other than the state of the connector itself). If the bulb lights up when the handbrake is on (with the wires to the connector disconnected, then you know the bulb & handbrake switch works. (If they then don't you either have a bulb failure or a poor connection to the handbrake switch - to get to the handbrake switch you have to take the driver's seat out)!

Cheers,Charles
1980 Roadster

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Charles Farran
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Re: Bare "plug" near brake master cylinder

Post by Charles Farran » Fri Apr 24, 2020 12:51 pm

20200424_124041.jpg
See extract from MGB repair manual.
Cheers,Charles
1980 Roadster

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Michael Barclay
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Re: Bare "plug" near brake master cylinder

Post by Michael Barclay » Fri Apr 24, 2020 6:16 pm

Hi Dave
After reading all the reply’s it sounds like all the rest of the braking system is working so if it’s not causing you any problems then I would leave it alone . After all all this stuff was only added to the cars to meet the US market safety regs.
Best regards
Mike Barclay

Dave Wheatley
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Re: Bare "plug" near brake master cylinder

Post by Dave Wheatley » Fri Apr 24, 2020 6:59 pm

I suspect you are right Michael regarding the US regulations.

Anyway, I've managed to * (censored!)a self tapper and fibre washer into the remains of the switch left in the master cylinder as a token backup.

I must say it was a relief to discover that brake fluid does not touch the workings of the pointless switch, the remains of which are back hidden behind the master cylinder where I found it.

After the success of that, I set to and replaced the offside rear hub oil seal which has been leaking a bit for a while now. Piece o' cake!
1978 MGB GT
1931 Austin 7 RM
1955 Standard 10
1972 Ford Cortina
No modern!

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