bleeding brakes

Technical MGB discussion
Vic Butler
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Re: bleeding brakes

Post by Vic Butler » Sat Oct 03, 2020 3:56 pm

I removed the broken switch using a mirror, torch and tapped the remains with a thin screwdriver. It came undone easily.
As Charles said the switch is available.
I suspect if the car was MOT'd it might fail as the light wouldn't be working when the handbrake was applied unless that didn't apply due to the age of the car.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

Not_Anumber
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Surname: Silk

Re: bleeding brakes

Post by Not_Anumber » Sat Oct 03, 2020 4:11 pm

I went to undo the pressure relief switch by 3 1/2 turns but it had already been loosened and only took a turn to remove. As others have said there was no brake fluid leakage when removed.

Does anyone know whether I can now go ahead and bleed the brakes with the switch fully removed or whether the switch has to be fitted at exactly 3 1/2 turns away from tight?

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Charles Farran
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Re: bleeding brakes

Post by Charles Farran » Sat Oct 03, 2020 4:53 pm

The brake warning light will come on when the handbrake is applied & the ignition is on whether or not the brake warning switch is connected up or not.The light will only stay on if the warning switch is connected up with the plip button extended as it would be if resting on the differential bar (but not in the two areas where the indents are. To bleed the circuits properly,the bar must be able to move which it can't if the switch is fully wound in and the plip is wedged in one of the indents.In theory you could take the switch out altogether when bleeding & then put it back in again & then check if the warning light stays off when the ignion is on & the hand brake released.
Cheers, Charles
1980 Roadster

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Charles Farran
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Re: bleeding brakes

Post by Charles Farran » Sat Oct 03, 2020 4:59 pm

You may want to check that the switch functions electrically by removing it and pressing the plip in to see if the wzrning light goes off (handbrake off of course) ignition on ,otherwise you will never get the result you are looking for!
Cheers, Charles
1980 Roadster

Not_Anumber
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Re: bleeding brakes

Post by Not_Anumber » Sat Oct 03, 2020 6:08 pm

Thanks, i very much appreciate the information. I've made up a simple test harness using a length of heat shrink tubing over each of the prongs on the pressure relief switch which also traps a thin pair of wires to run to the multimeter. Ive checked the switch is open circuit when pressed in and closed circuit when released. Now to bleed the brakes again and refit the switch to. The wires can later just be pulled out after that and the car's connector plugged back in. I'm several days away from being able to use the car's electrics to test as the loom in the engine bay is disconnected at most points at present

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Charles Farran
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Re: bleeding brakes

Post by Charles Farran » Sat Oct 03, 2020 6:27 pm

I have just reread my post of 4.53. What i meant to say ( which you have realised ) is that with the hand brake off the warning light will stay on if the plip of the switch is extended because it is lodged in one of the indents of the differential bar whereas if the circits are properly balanced the plip will be depressed as it will be resting in the middle of the bar where there are no indents. (Apologies if i am being pedantic)!
Cheers, Charles
1980 Roadster

Vic Butler
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Re: bleeding brakes

Post by Vic Butler » Sun Oct 04, 2020 8:27 am

Not Anumber. Bleed the brakes exactly how the workshop manual says otherwise there's a chance the piston will be decentralized. They must be bled in the correct order and with the correct pedal action.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

Not_Anumber
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Re: bleeding brakes

Post by Not_Anumber » Tue Oct 06, 2020 1:15 pm

Does anyone know if there is supposed to be a washer for the pressure switch ? There was a poorly fitting fibre washer on mine that i had doubts about. I went to order a replacement from MGB Hive but they said no washer seems to be specified and they wouldnt expect there to be a requirement as no brake fluid can escape from there.

Is this ok ?

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Charles Farran
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Re: bleeding brakes

Post by Charles Farran » Tue Oct 06, 2020 3:56 pm

Hi.
There is no fibre washer. The warning switch simply screws in although i found it quite tiresome when threading in a new switch as the threads in the underneath of the master cylinder body are very fine & in my case clogged . I used the old broken switch to try & dislodge the grime before fitting a new one - the first few turns getting it start was challenging! The switches are not particularly robust & bought a spare shortly afterwards as I feel next time i have to remove the one in place, the plastic threads may get damaged and / or the body of the switch split.
Cheers,Charles
1980 Roadster

Not_Anumber
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Re: bleeding brakes

Post by Not_Anumber » Sat Oct 10, 2020 5:42 pm

After wasting another 2 hours not being able to reliably bleed the rear brakes I checked it at the cylinder unions and found it is hardly putting out any fluid at all to the rear circuit though healthy at the both the fronts. A new master cylinder is now on order from.the MGB Hive.

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