Replacing brake master cylinders

[265bhp]

Hi all

I have to replace both master cylinders on my 63 FHC as the originals are completely rusted through...I have the chance to buy a pair of Dunlop cylinders, apparently used on a competition etype before..but they are 3/4 bore not 5/8, with numbers Q Dunlop VBM 4664 and 466

Would anyone know if there is any reason that I shouldn't use these

Many thanks

Jonathan

[Heuer]

Check out our Knowledge Base - there is a link to a very good article written by Mike Cassidy on overhauling the Kelsey Hayes system which may give you some guidance.

[265bhp]

Thanks David

Great articles, very useful as I will be overhauling the brake system, sadly no reference to whether larger bore cylinders can be used

Does anyone else have any experience please...?

Regards
Jonathan

[Heuer]

The larger diameter 3/4" cylinders are going to give you more braking force. Whilst this may be desirable on a race car with wide wheels and sticky tyres running at high speeds you need to be circumspect of using that set-up on the road. You may find at legal road speeds and normal tyres the car is over braked and you start locking up the wheels, particularly in the wet. In my experience there are few, if any, race car modifications that work well on a road car. Personally I would stick with the standard cylinders and carefully restore the servo system as per the excellent Mike Cassidy guides published in the E-Type Club magazine.

[MikeC]

I did not cover 3/4 inch master cylinders in my article because I was only covering the standard restoration of the Kesley Hayes unit though I have come across one instance of a car fitted with such units which are possibly originally clutch units.

Although I have no direct experience the important point to remember is that Jaguar in their 3.8 brake design fitted different front and rear master cylinders presumably because of some front to rear balancing reason though any such reasoning as to need was not carried through to the 4.2 as far as I can tell.

Although identical externally the standard 5/8 inch master cylinders have completely different internal pistons and I believe (though I have not measured it) different length push rods even when the upper adjustable push rod is adjusted to give the total 1/16 inch free play.

The result is that the front master cylinder stroke is 1 and 3/8 inches and the rear has only a 1 inch stroke (Jaguar Workshop Manual). This means that more fluid is sent to the front brakes than to the rear. Not many people appear to be aware of this and parts suppliers have been selling rear master cylinders for use on the front position with instructions that only the push rod needed to be changed which is bad advice meaning that the designed front to rear ratio is lost.

If replacing the standard 5/8 inch correct master cylinders with any none standard units including 3/4 inch units this designed in balance would also be lost as with incorrect fitting of standard units either reversed, two front or two rear units.

As far as I can tell at a basic level fitting larger bore master cylinders would simply mean that the slave cylinders would extend to fully on with less movement of the pedal and lead to a full on brake pedal simply being higher which may or may not lead to a less ideal foot position. In a race application with larger slave cylinders the cavities would be larger and require more fluid.

I do not think it adds any "power". however the Jaguar designed "front to rear" differential would be lost. whether that matters (changing the amount of fluid to front and rear slave cylinder ratio) unfortunately I cannot say.

Best,

MikeC