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