Master Cylinders (Oh please, not again!)

[surfguitar58]

I know the brakes topic has been  elsewhere, but I came across this illustration that is EXACTLY the Girling dual circuit MC that is standard equipment in the Fed TCS. BUT, now I am really confused!

The illustration clearly shows the stepped bore of the cylinder (.875" stepped down to .812" IIRC) which I always thought was a way of reducing pressure to the rear drum brakes without the use of a proportioning valve. But if the seal on the right hand piston is assumed to be frictionless, and we ignore the springs, the pressure on both sides will be equal, right?

The springs do have some effect. The right hand spring is stiffer than the left so it effective reduces the pressure slightly (5 to 10 PSI depending on travel) in the right hand chamber, but that is the front brake!?! Ow, my brain hurts. 

Can someone straighten me out? Are front and rear brakes the same pressure or not?

Tom

[Pfreen]

The pressures are equal. However, the force exerted on the smaller piston is reduced by the piston area ratio.  So, I don’t think the stepped bore makes any difference to the rear brakes.

To visualize this, assume the primary piston area is 1sq in and the secondary piston area is .862 sq in.

If you apply 10 lbs on the pedal, the pressure generated is 10/1= 10psi.  The force acting on the secondary piston is 10 psi* .86 sq in = 8.6 lb.  The pressure on the outlet side of the secondary piston is 8.6 lb /.86 sq in = 10 psi.

[Lotuswins]

Yes, pressures are nearly equal especially when under a panic stop.  The secondary piston is a pass through since there is no diameter change.  Note also that the seal on the secondary piston is a double lip seal, so it seals both ways.  I think the smaller bore though is smaller than .812, since the .700 bore S2 cylinder is identical except for the primary size bits.  So which outlet for the drum brakes?  I believe the secondary outlet is for the drum brakes since the output volume is much less, and the drums volume need is much less also.  That way if one circuit fails, the other would match the needs of the remaining circuit.   I believe that is also illustrated somewhere in the manual, but that is from memory....and you know how that goes.....

I once had issues with brake performance and swapped front to rear to see if I could get better front brake pressure, and there was no detectable change....then I looked closer at the construction of the MC, and there it was....

Jerry Rude
4005R