How 3.8 Brake Master Cylinders Work (Without the Smoke and Mirrors)

[rfs1957]

Hi Marek,

Because of what has been written up until now (and constantly referred to) about "hydraulic differentials" and "moving liquid" I had been unable to find anything published that related to the true behaviour and function of the 3.8 linkage, where - had the pivot not been bang in the middle of what is indeed a "no-bias" bar - there would be a bias in favour of one or the other.

The use of "two masters connected directly to the brake pedal" is not a prerequisite for getting a bias.

Otherwise referred to on the 3.8 as the Balance Link Assembly C.20341/C.17244, the "no-bias" bar happens to be symmetrical, but it would be perfectly possible to offset the centre attack point and NOT get 50/50.

If a diagram would help to illustrate this point I would be happy to oblige.

[bopperd]

Rory,
Thanks a ton for the dissertation on the difference between upper and lower cylinders. I am rebuilding mine right now and, sure enough, the pistons were reversed. The brakes on my car, 876024, had behaved a bit oddly but I wrote it off as a side-effect of the fat, sticky tires I had on the car. Looking forward to experiencing the difference.
Dave

[cactusman]

Well all I can say is....Brilliant Rory. Makes perfect sense.

Maybe this could be put on the knowledge base? All I need to do now is see just what I have in mine...suspect it won't be what it should be!

[rfs1957]

I wasn't able to restore every single picture shown in these posts, but the essential gist is there.

[Philk]

I wasn't able to restore every single picture shown in these posts, but the essential gist is there.

Has this been whittled down and added to the knowledgebase?.... I could not find it

[mgcjag]

Hi Phil.....as far as im aware its not in the Knowledge base section as a seperate artical......but does it really need to be as all the other posts makes it into a great thread...our whole forum is a knowedge base...however it would need to be pulled together and tidied up into an artical....but just needs someone to do that....then contact the Admin (Heuer) to ask if it could be added....Steve

[MarekH]

Call me old fashioned but can't we just ignore 99% of this thread and say the purpose of the pivot arrangement is to make the hydraulic pressure in both circuits equal to each other. If it isn't, then the pivot simply pivots about a bit until it is.

kind regards
Marek

[tinworm]

The way I think of it is there is a certain amount of lost motion in each circuit (until all pads on all calipers on both circuits are hard up against the discs) the balance link allows the system to find its initial position (and compensates for any differences).

Barrie

[Gfhug]

Not having a 3.8 my comments may have the same value as a. n. other in a similar position.
But what I can say from reading this thread and from other postings by Rory (rfs1957) in other areas is that he obviously has very high automotive engineering knowledge and experience that he puts to exemplary use by helping to explain to those less knowledgeable how systems work.
I've certainly gained a lot from reading his posts in other areas and benefitted from his generosity to fellow E-Type owners.

Geoff

[PeterCrespin]

Call me old fashioned but can't we just ignore 99% of this thread and say the purpose of the pivot arrangement is to make the hydraulic pressure in both circuits equal to each other. If it isn't, then the pivot simply pivots about a bit until it is.

kind regards
Marek

You’re old-fashioned.

HTH

Pete

[MarekH]

HTH? - I had to look that up. It sort of proves your point.

I also agree with Barrie - the see-saw is sure to move if the free play is uneven as the pedal is first pressed, then it ought to just float to try keep the line pressure the same, were it to become different for any reason.

kind regards
Marek

[288gto]

.........can't we just ignore 99% of this thread..

kind regards
Marek

Alas that would mean skipping past all the lovely pictures, diagrams, jigs, write up and comments straight to Peter calling you old fashioned.

I can't help thinking the substance of the original thread would be somewhat lost.


Simon

[cactusman]

To me the most important part of this thread is the first post which, to me at least, seems well researched and correct. I can't fault Rorys excellent post.
My own view is the Kelsey Hayes system, while maybe not the best, is unfairly maligned by at least some people in the wider jaguar fraternity. Looked after it works perfectly well. The Lockheed system can provide more boost to the pedal making it feel better but that's about it.
My system works well. Although I have fitted coopercraft front callers the rears are standard. As said, and this applies to any servo system, the servo just adds effort to supplement the drivers foot. It means you need less foot pressure but does not intrinsically improve the brake efficiency. That is a product of pad size, pad material, disc material, hydraulic pressure and brake temperature. If the servo fails the brakes "feel" less efficient simply because your foot now has to supply all the brake pressure and also has to overcome the now dead servo....which is entirely possible if you can push hard enough!
Just my thoughts and I do not claim to be an expert...

[rfs1957]

Some of the links to the drawings and photographs for this article had evaporated over the years, as someone has just kindly pointed out.

I have re-established them, and if anyone else had been frustrated by this, it's worth taking another look.