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109 Dual Circuit Master Cylinder w/ 10'' Drums


wood-gee

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I currently have a 10'' SLS single circuit brake set up with non servo master cylinder (MC) on my series 3. I picked up a 109 servo pedal box with dual circuit MC last year to put on at some point, and as I'm in the process of installing a 200 tdi with a good vacuum feed it seem a sensible time to fit it. I'm on quite a budget, so was hoping to fit the MC and converting the system to dual circuit (swap the 4 way splitter to a 3 way splitter for front, then direct line to rear brakes). I've spent a lot of time trying to read about the topic on a variety of forums, but as far as I understand, the difference between an 88'' and 109'' dual circuit MC is a 7/8'' vs 1'' bore. The risk of using this is the increased flow will result in little to no modulation in the pedal, but equally, there's a good chance this might not be an issue.

I'd really appreciate it if anyone could offer any wisdom on the matter, or if anyone has any first hand experience of such a set up.

As a further question, my servo currently hisses out of the front seal when the pedal is depressed (and sucks when it's brought back up), I'm assuming this is not normal and the seal needs replaced?

Thanks

Jake

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I'd first suggest that you get a S3 parts book - who knows - there may even be one that can be downloaded.

Bigger cylinder gives a heavier pedal - its curious that many S3 diesels were built with the servo as standard - and I wonder if it had more to do with the time between adjustments that a light pedal. Certainly my S3 SWB with servo, dual circuits and 10in brakes didn't seem to stop any better than a S2 without the servo.

The parts book describes master cylinder NRC 6096 for dual line systems - and makes no reference to specific vehicles.

http://www.paddockspares.com/nrc6096-brake-master-cylinder-dual-line-servo-no-stamped-is-74660213.html

There are further references to the dual line system fitted to earlier S3's in the optional parts book (It was an optional extra - as was the heater and the canvas hood - so no reference in the standard parts book!)

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NRC6096 is what I used on my 88", which got a dual line system when I rebuilt it. I used the 11" TLS setup on the front as that's what post-rationalisation (c. 1980?) 88" models used. I used a pedal box/servo off a post-1980 88" too. It stops very well, but then I never drove it before it was rebuilt so can't really compare.

I think (ready to be corrected though!) that on rationalised vehicles the servo/master cylinder was common between wheelbases, along with the front brake setup. The exception being the 109 V8 which had the wider front drums/shoes.

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Thanks second jeremy, that's exactly what I was keen to hear re the stopping vs a non servo vehicle, and what you're saying would concur with this thread I read on this >> http://www.series2club.co.uk/forum/forum/index.php?topic=22759.0 , which is also where I got the info about bores from. I had actually looked at a parts book, but as you mention, it only details the LWB MC in the main parts book.

I suppose it makes sense with bigger bore - heavier pedal, given the whole Pressure = force/area, so you're generating less pressure for the same force.

It's helpful to hear your's stops really well with a similar set up too retroanaconda. I think where I differ is that I don't have, and don't intend to fit TLS brakes up front, so by using the LWB MC I may not see much improvement between it and my current set up, which I'm happy with, as I like the idea of the safety aspect of a dual circuit system, and I can always change the master cylinder at a later date when I'm a little less skint which would in turn improve it further one would hope.

Thanks for the help guys, it's helped me to finally make my mind up on what to do!

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Thanks Serious series, that's a good point, I'd just kind of assumed that the tls slave cylinders had a bigger bore, but glad that I have now checked - the bore diameter of a TLS slave is 1 1/8 compared to that of a sls front which is 1 1/4, which may well lead to issues as you've mentioned. Meaning that I may have to stump up and get a different master cylinder after all, but better that than something far worse for having incorrectly set up brakes.

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All standard Land Rover master cylinders have a single diameter bore. The differential pressure between front and back cylinders being achieved by the use of larger diameter cylinders on the front (or smaller on the back if you prefer). Looking at the cylinders its easy to identify the back cylinders as they have a flat in the casting above the round bit.

The larger cylinders (front on 10in installations) are used at the back on long wheelbase vehicles - which of course have 11in brakes and twin leading shoe brakes on the front - so its possible that asking for 'back brake cylinders' could produce large ones if the supplier doesn't realise that there is a difference between long and short vehicles.

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Had a bit of time today so did some calcs of cylinder volumes.

For 3/4 master cylinder on 10" SLS set up 4.6mm of master piston travel moves each piston 1mm

For 1" master cylinder on same set up only 2.6mm of travel needed

Not so simple to calculate volume for dual circuit master as connecting rod reduces volume of fluid being moved in that section of master cylinder .

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