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Brake master cylinder compatibility on a S3


Dan110
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Hi there, any input appreciated...

I'm replacing the brake master cylinder on a 1972 S3. It's fitted with the 'Centre Valve' type at the moment and i'm wondering if the Compression Barrel type can be fitted as a direct replacement (with associated change in the pipework, obviously).

Worth the change? Or no advantage?

Thanks

Dan

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Things I forgot to clarify:

It's a 2 1/4 petrol and i'm wondering about changing from the earlier unit which has a single output which branches at the chassis, to the later unit which I believe has two outputs - one for the front and one for the rear. The reason for the change would just be for the added safety factor of retaining some braking if a pipe were to fail.

Cheers

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Have you a servo?

I think the later (post '80) models had a different brake pedal so am unsure as to whether the later barrel would fit. A good trawl through the parts manual would shed some light on weather your pedal would work.

Also, would you want a PDWA ?

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Thanks for the reply.

No servo and i'm thinking about leaving it like that. I presume the pedal difference is related to the servo - different pivot point maybe?

Anyway...A bit more googling and it seems as though there's no real difference between the two other than the outputs exiting separately - i.e. Still all being pushed by one cylinder so no separation between front and rear. That being the case, i'll just replace it like-for-like and leave it at that.

Hadn't thought about a PDWA - is that a simple addition?

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I think the mounting holes in the flange for a CB cylinder are vertical and those for a dual cylinder are horizontal.

More use than a PDWA would be a reservoir cap incorporating a level switch. Brakes seldom fail from major leaks - the usual problem is slow leaks past seals and the level switch would alert you of the situation.

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I think the mounting holes in the flange for a CB cylinder are vertical and those for a dual cylinder are horizontal.

Thanks for that. And fair point on the modes of failure - Think I'll get a standard cylinder and look into the options for an added layer of safety once the list of jobs is exhausted!

Cheers

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I can't remember the mounting flange orientation of the servo equipped MC, but I agree that leaving out the PDWA valve is a good idea - they make bleeding more difficult, ar a likely source of any leak they detect, and will probably not detcect a leak anyway as they are not sensitive enough when working and re prone to gumming or rusting up anyway. The level sensing cap from a Defender, Discovery or RRC is a direct swap, you just need to connnect the wiring.

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The caps on my SIII brake reservoir and clutch master, the Lightweight brake reservoir and master, my 95 RRC brake reservoir and clutch master and the Defender units are all the same, so I would assume that'll fit perfectly.

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  • 4 weeks later...

Leave out the PWDA. Its extra plumbing and was only there because some lawyer said there had to be some indication that there was a brake failure. Unfortunately, it only actuates AFTER you step on the brake pedal and by that time you foot should have told you there is a problem.

I just recently converted my IIa from the single circuit to a dual circuit system - but I went with the servo-equipped master. And its a new pedal tower with a bigger hole in the bulkhead plus modifying the wing to accomodate the servo..

And more fun, I'm in the USA, its a RHD truck. I had some fun finding another servo-equipped RHD truck to make a pattern for the modifications.

I had looked at options for a non-servo dual master - I may have been able to find one if I had a part number...

Cheers

Mike

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Standard dual line MC are for servo systems and the fittings are rotated 90 deg for early systems and 45 deg for later systems.

If you want a non servo dual line the only MC I could find is a Wilwood 1" MC, this has vertical mounting holes.

The MC is 7" from mount to the end and would hit the bulkhead so a hole is required which would need a cover plate afterwards to maintain the firewall idea as a non servo system has the MC facing backwards and a servo MC faces forwards which is why the pedal box tower thingy has a top hinge and the non servo box has a more central hinge.

I am looking at doing this as I have an S1 so no room for a servo pedal, I have two remote servos to fit ala Volvo C303.

Marcc

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