Brake Master Cylinders

[Simonpfhc]

Hi all,

I have been following this forum for a couple of months now and I have finally got an issue that I need some help with.

I am the proud owner of a full marching numbers 62 FHC that I bought back in August and have started on a full nut and bolt restoration - it will be long project!

Anyway, I have just finished stripping and rebuilding the pedal box and have identified a problem with the brake master cylinders - both have the piston assemblies designed for the rear. I have read with interest the 4 articles by Mike Cassidy without which I would never have discovered this issue, or even been able to re-assemble the components correctly (thank you David!). It looks like all the major suppliers can only supply what looks like brake assemblies designed for the rear??

So two questions - where could I obtain the correct front master cylinder piston assembly, and secondly, how can I be sure that the internals being supplied are correct before I order one?

Next, not really a question, more of an engineering query, is around the purpose of these different piston designs for front and rear master cylinders. Mr. Cassidy states that the longer stroke piston of the front MC allows more fluid to be pushed to the front brakes effectively creating a mechanical brake bias of about 66f/33r.

What I don't understand is how a 'longer' piston can move more fluid? A bigger bore certainly would, but the rear of a longer piston will move exactly the same amount of fluid as the rear of the shorter piston. The only difference is that the cylinder with the longer piston will hold less fluid in total. If someone with more engineering talent than me could shed some light on this, I would be extremely grateful ;-)

I have already started taking loads of picture of my (slow) progress and also started a rebuild diary. One day I will get around to setting up a blog for others with similar interest to peruse........

Looking forward to seeing what replies come back ;-)

Simon
62 3.8 FHC

[abowie]

Looking at mine the actual piston is pretty simple; I reckon someone with a lathe could knock you one up.

Pics here:
http://www.flickr.com/photos/71499700@N ... 502542509/

[abowie]

The long piston goes with the short pushrod.
The long end of the long piston is 16.25mm from the seal groove to the end.

The short pushrod is 42.50mm long.

Comparison picture on Flikr as we..

[MarkE]

Simon, I have used Classicar Automotive in Cheshire for over 20 years now for brake components for Jaguars, Healeys and Lotus. They really know their stuff, and provide a very cost effective reconditioning service, as well as component supply.

I used to overhaul all my own brake assemblies using their components, but once I saw their reconditioned units, I had to use that service.

They know what to look for when examining our 50 year old components, and properly clean / de-rust / replate as required, and on completion of the rebuild, pressure test the final assembly. A bit safer than my approach.

I had three brake failures in old cars last year, all with old components, and one that I had restored 12 or so years ago. So I'm on a bit of a crusade!

A chum recently wrote his Lotus Elite off 6 weeks after overhauling his master cylinder. He had spent over 1000 hours restoring the car, and felt that popping ?2 worth of seals in the master cylinder was good enough. Turned out that the cylinder had a fine crack, which destroyed the seal very quickly, and hence the car. And he's no cowboy restorer, having built many race Lotus cars and Coventry Climax engines. It's just that without a pressure test, he couldn't know how good the cylinder was going to be.

Anyway, try Classicar Automotive, as they will be able to advise you on components or overhaul.

[Simonpfhc]

Andrew, Mark, thanks for your replies, really appreciated.

Andrew, thanks for the detail measurements of the correct firment piston - it may well be easier to get someone with a lathe to make one up.

Mark, I will certainly give your recommended contact a call - sounds like good advice to me.

Well, my first post has proved fruitful ;-)

I think I will need to PM David Cassidy for some more engineering insight as to how the piston affects the brake balance ;-0

Cheers.

Simon.
62 3.8 FHC