Master cylinder question.

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DaveEFI
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Re: Master cylinder question.

Post by DaveEFI »

Early Minis with all drum brakes had a balance valve, which reduced the rear brakes too. I had an 1800 Land Crab with front discs which did so too - but had a gravity operated one, same as a seatbelt. The SD1, having dual circuit brakes just makes it more complicated. Early cars were front/back split. Later ones - like mine - have the calipers split into two. So either 'half' the front brakes only, or half plus rears, depending on what fails. Hence the odd pipework round the balance valve. I think. :D


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harvey
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Re: Master cylinder question.

Post by harvey »

DaveEFI wrote:
Sun Aug 08, 2021 11:29 am

No electrical connection to the balance valve on the SD1.
Oh, that's my memory failing me then, (again). Must have been something else current BL at the time.

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Re: Master cylinder question.

Post by SuperV8 »

DaveEFI wrote:
Thu Aug 05, 2021 1:24 pm
I'll say how I think the balance valve works. The BL manual ain't much help as it really only deals with the early system. On that, it handles front and back as separate systems from the tandem master cylinder. And reduces the pressure to the rear drums to help prevent premature locking.

My late car (Vitesse brakes) has split feed calipers. One side of those is fed, via a T, from the back output of the master cylinder. So doesn't go through the balance valve.

The other master cylinder output is T'd and feeds both the balance valve inputs.

This drawing may help:-

Image
Image

This might be easier to understand/visualise - from a RangeRover but similar system except for the rear discs in lieu of drums.

The front brakes are actuated by two independent hydraulic systems, so that if the primary system should fail a secondary circuit is always available.

Should there be any leak in the primary circuits the dual master cylinder ensures the maintenance of pressure in the secondary circuit.
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DaveEFI
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Re: Master cylinder question.

Post by DaveEFI »

It's likely how the SD1 should be done. But because it was altered during the production run, the second front circuit doesn't go near the balance valve.
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GDCobra
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Re: Master cylinder question.

Post by GDCobra »

DaveEFI wrote:
Mon Aug 09, 2021 3:59 pm
It's likely how the SD1 should be done. But because it was altered during the production run, the second front circuit doesn't go near the balance valve.
It's also interesting that they term it a "Pressure Differential Switch" rather than a balance valve, this was my initial thoughts on it. It is there to detect a leak and isolate the offending circuit rather than to limit the pressure in one of the lines. Maybe that is the difference on later vehicles.

With drums on the rear it probably doesn't need pressure reduction unless the 4 pot fronts are equally poor.

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Re: Master cylinder question.

Post by DaveEFI »

Think it's the self servo action of the leading shoe on drum brakes that requires the pressure reduced to them. To stop them locking up first.
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stevieturbo
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Re: Master cylinder question.

Post by stevieturbo »

GDCobra wrote:
Mon Aug 09, 2021 4:14 pm
DaveEFI wrote:
Mon Aug 09, 2021 3:59 pm
It's likely how the SD1 should be done. But because it was altered during the production run, the second front circuit doesn't go near the balance valve.
It's also interesting that they term it a "Pressure Differential Switch" rather than a balance valve, this was my initial thoughts on it. It is there to detect a leak and isolate the offending circuit rather than to limit the pressure in one of the lines. Maybe that is the difference on later vehicles.

With drums on the rear it probably doesn't need pressure reduction unless the 4 pot fronts are equally poor.
On Minis, it was simply a valve to limit pressure sent to the rear wheels. Not a balance valve.
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