Jag Master cylinder question

[Snake2998]

[IMG] https://photos.google.com/album/AF1Q...MRvsLWAgrHBvlH [/IMG]

Can anyone explain the science of how the Jag master cylinder works. I am trying to understand it to see if I can get any more pressure to the rear callipers.
As I understand things, when one brakes the servo pushrod pushes the primary plunger (#9 in the attached image) forward this does three simultaneous things:

o It compresses the brake fluid and forces it flow to the front callipers initiating front braking.
o It compresses the intermediate spring (#10) which starts pushing the secondary plunger (#12) forward to activate the rear callipers.
o The pressurised fluid in front of the primary plunger adds to the activation of the secondary plunger.

What I don’t understand is what the function of the intermediate spring is and what the impact on the rear brake pressure would be if one either stiffened it or softened it.
I am trying to increase the rear brake pressure as my car (and possibly all Jag based cars??) is significantly under braked at the rear.

ps - I hope the photo comes up

[Tom Wells]

Snake,

Your link doesn't work.

Plus, my Jag brake-equipped Cobra has excellent front-to-rear brake balance, as did my 1965 3.8S which had a very similar braking system.

Here's a reference which explains the front-to-rear brake balance as set up either by the factory, or by you if you use an aftermarket proportioning valve as I do in place of the factory unit. Look on Wilwood's site or Baer's site or similar to see some of what's available.

Link: https://auto.howstuffworks.com/auto-...ter-brake2.htm

Hope this helps,

Tom

[Snake2998]

Thanks for that Tom - unfortunately my experience, using a brake calculator and upgrades to the front brakes both show that the rear is in fact way under-braked.

The Jag system is designed for a nose heavy weight distribution and much more forward weight transfer under heavy braking with the softer springs than the Cobra which also has significantly larger rear tyres than the front.. My Cobra has 45/55 rear biased weight distribution compared to the jags 55/45. The jag also weighs almost 50% more than the cobra..


An acquaintance of mine has put a pressure limiting valve on his Cobra to limit the pressure to the front brake so that additional pedal pressure will feed more pressure to the rear and he says it has improved the stopping ability of his car significantly and is still unable to lock his rear brakes because the standard rear caliper is too small for a Cobra. This is totally contrary to all all recommendations for the use of brake pressure limiting valves and Willwood would freak at this use, but does seem to work! He says it works well on both road and track

My calculations support his findings but, if I can't modify the MC I might well pull the pistons out of the rear caliper and cut it up to preserve the handbrake and fit a second front caliper from some car of suitable piston size and then fit a pressure regulator on the rear to limit lockup.

[joyridin']

What style caliper does the rear brakes use? Jaguar had a couple different styles through the years. The early style with bolt-on pistons were real easy to modify. You just find a set of front piston pods and bolt them on. Later years, the calipers were 2 piece and single piece and it is a bit harder. Just adding larger calipers is going to make the situation worse though. Now the master isn't going to push enough fluid to correctly work the calipers. In reality, you would want a smaller piston diameter to initiate lock-up. Now it will take less fluid to activate the calipers and you can put an inline adjuster to adjust the bias.

This would probably be a bit more work than you are willing to put forward, but ditching the single master and going to dual masters will really help the situation. You can now adjust the bias any way you like.

[Snake2998]

Mine are the two piece MCs.

Yes I am also looking at other alternatives like twin MC's with balance bar. Another alternative is a to add a small disk to the propshaft to diff flange for a handbrake and new calipers for the back.

I will look at the early calipers as an alternative - wasn't aware of that option thanks.

There isn't much difference in the MC diameters front and rear so am pretty sure that it won't make much difference in capacity to drive larger pistons.

My car is an automatic so I am not too concerned about pedal pressure as I can easily do two foot braking as the pedal is big enough.

[joyridin']

Larger pistons are going to make the situation worse. If you want to increase braking to the rear in an attempt to initiate lock-up, you will need to use smaller pistons in the calipers. The MC will put more pressure on the rear causing them to react faster. I have .750" diameter master for the front and .625" on the rear with Jaguar calipers.