Stepped Bore Master Cylinders

Aka quick take-up master cylinder.
These things intrigue me...
I watched a video over lunch that basically said the big piston pushes fluid past the smaller seals and there really is no proportioning effect like I once thought.

Basically it functions as an overrunning clutch with the larger bore moving fluid until pressure trips it off. From there the two inner bore pistons move fluid to both circuits.

Now is where I am confused... how are two pistons moving inside the bore relative to each other? There's a bunch of springs but does the same volume exit each port?
How is this fail safe if fluid passes past the seals?

I got more questions than answers out of it...

 

The video you saw is probably about Master cylinder with a quick take up for disk brakes that retract the pistons for low drag.

Starting at slide 57 in this presentation explains that: https://slideplayer.com/slide/4616100/



The bigger piston overflows the primary piston in the front wheel circuit until pressure builds. Then it works the same as a normal dual master cylinder.

The Jeep stepped bore master cylinder from the '60s is not like that. The piston for the front wheels is 7/8" (and in the front of the master cylinder) while the piston for the rear brakes is 1". This will give a higher pressure to the front brakes.

You can see the different piston sizes in this picture from my '68. Rear brake circuit piston on the right and front brake circuit piston on the left:


I suspect the reason these Jeeps have the front brakes on the front piston is because you can't physically put a bigger piston in front of a smaller one in a single bore. Not sure why it's become a universal standard that most everything else has the front brakes on the rear piston.

As for how all the springs and pistons work, If you step back several slides in the above presentation, it shows how the dual circuits function and fail over.

For a verbal description, the descriptions below are relative the to circuit position in the master cylinder not what end the wheel circuit is.

• In normal usage, the fluid pressure of the rearmost circuit pushes the piston for the front-most circuit.
• If the front-most circuit fails, the piston bottoms in the end of the master cylinder and stops. Then the rearmost circuit can build up pressure against the bottomed up front circuit piston.
• If the rearmost circuit fails, the piston bottoms out on the front most piston to push it and build pressure in the front-most circuit.

Note: In the event of a failure you need to bottom out the failed circuit piston leading to long pedal travel. It's import to have enough travel so the pedal can build pressure in the remaining circuit before it hits the floor.