Tandem (aka Dual) Versus Single Brake Master Cylinders"Should I upgrade my brakes" Common question at most Morgan events or the GoMoG help Line for owners of older Morgans. Yet these cars had excellent brakes by the standard of their day and with their low weight, Morgans don't need much braking. However, there is a element that may point to a need for a SAFER brake system.
Most Morgans made before the 1970 will have single (piston) master cylinders (MC). A leak in that systems brake plumbing (not uncommon with any system), happening anywhere on the car, can render these brakes completely inoperable, leaving one to rely on the Morgan hand brake and luck. If you have one of these earlier cars,
upgrading to a tandem master cylinder is very wise. You see a tandem master cylinder has TWO (not one) two pistons operating in tandem (one after another) in a common piston chmber and bore as shown in the diagrams.
N.B. In rear wheel drive cars (Morgans) the piston and its MC outlet that's located closer to the pedal (piston 1) activates the front brakes. The fluid pressured by this piston (1) also causes the movement of a second piston (piston 2). (Piston 2 applies the cars rear brakes).
The diagrams show how tandem master cylinders will isolate the effect of a leak to the subsystem (front or rear) it appears in.
A LEAK IN THE FRONT will not affect rear brakes and a leak at the rear will not affect front brakes. As can be seen from the first diagram, if a leak develops in the front brake system, piston 1 will move forward until it contacts Piston 2 (and its seal which isolates the hydraulic effect). Force from the brake pedal will be transmitted mechanically to piston 2 via piston 1. Braking performance will be compromised, but the rear brakes will still be functional. The pedal will need to travel further than normal to fully engage the rear brakes. Braking will be difficult iof only the rears are working but at least you HAVE brakes.
UPDATING THE MC
To update from a single to tandem brake master cylinder,
one can try to source the .75" bore Girling unit used from the 1970s to
1993. This will be very diffiuclt as the unit is obsolte now and no longer
in prduction. When they WERE available before 2004, they were very dear,
sold by the MMC for 335£ ($550 USD) each. However, there are alternatives.
The .75 bore seems to be a happy compromise but it is bigger than the earlier
single master. The larger diameter piston displaces more fluid per
unit of travel so less pedal travel will be required to create the same
effect at the wheels. Pedal effort will also be greater for the same effect.
However, so far, no one doing a swap to .75" has been unhappy with the
difference. If brake effort is a problem, owners can consider the present
MMC master cylinder with its integrated booster.