Dual Master Cylinder coupled with a Differential Valve
Figure 1
The original dual piston master cylinder fitted from 1970 onwards, operated with a differential brake valve, which incorporated a switch, that activated a warning light, alerting the driver of a leak in one of the brake circuits. A differential valve is not to be confused with a proportioning valve, which is often used in conjunction with a differential valve. Under heavy braking, a proportioning valve reduces the braking force on the rear wheels as weight transfer increases force on the front ones. This prevents the rear wheels from locking up. When Morgan fitted a differential valve, they did not fit a proportioning valve. I assume the braking force of the 11 inch front disc brakes are quite reasonably balanced with the rear drum brakes. The stiff front suspension on a Morgan also resists nose dive during braking. However rear wheel lock up does occur under heavy braking and care is needed to avoid losing the back end.
The front and rear circuits of a dual master cylinder feeds the opposite sides of the differential valve as illustrated in Figure 1. A shuttle piston situated in the middle of the valve separates the front and rear brake circuits. In a healthy braking system, when the brakes are applied, the shuttle piston senses equal pressure from both sides and does not move. However, if one of the circuits develops a leak, the piston will sense a lower pressure on that side and will move from its central position. The leaking brake circuit will be isolated and an integral switch will be actuated, illuminating a warning light on the dash board.
The Stop Lamp Switch is another device in the system which senses an increase in hydraulic pressure when the brakes are applied. A diaphragm in the switch shorts out a pair of contacts, which allow electrical current to flow to the stop lamps. Unfortunately the switch often fails and needs replacing. Always keep a spare and check the brake lights illuminate under light brake pedal pressure. See Brake Switch under the "Brake System" menu for more details.
The Dual Master Cylinder with Integral Warning Light Switch.
Figure 2
Since 1970, Morgan began fitting dual master cylinders, but the hydraulic circuit layout has varied and different methods have been used to improve the system. The original dual master cylinder fitted to my 1972 Morgan was always reliable. However as I have mentioned on the home page, my Morgan spent quite a long time in poor conditions after being damaged in "The Great Storm" of 1987. By the time I began restoring it, the brakes had siezed. Restoring the rear brakes presented no problem at all. New shoes and pistons were readily available. However the disc calipers were siezed solid and had deteriorated. New ones were expensive so I sent the old ones off to a company who restored them. It was relatively cheap compared with buying new ones and when they had been refurbished, they looked better than new. The bodies were cadmium plated and the pistons were replaced with stainless steel ones.
I decided to restore the the original master cylinder myself. When I contacted the Morgan Motor Company for a kit, they recommended that I should scrap the old master cylinder and fit a new one, which incorporated an integral warning light switch. The differential brake valve was no longer required. These often had to be reset when bleeding the brakes, which complicated a normally simple job. The new master cylinder with integral warning light also simplified the hydraulic circuit layout as illustrated in Figure 2.
The Diagonally Split Braking System used on Modern Cars.
Most modern cars are designed with a diagonally split braking system. It is important not to use this layout when rebuilding a Morgan. The swivel angle on Morgan front suspension is only 20 and camber angle is also 20. Therefore the angle between the two (included angle) is only 40. This results in the swivel axis intercepting the road inside the centre line of tyre contact. This suspension geometry is known as "positive Scrub radius", as shown in Figure 2 .
If half of the diagonal split braking system fails, only one front brake, along with the diagonally opposite rear brake, will be applied. A car with positive scrub geometry, will tend to turn around the braking front wheel. Under the same circumstances, negative scrub geometry produces torque in the opposite direction. This gives the driver a better chance of braking in a straight line. Negative scrub also reduces the chance of spinning in the event of a front wheel blow out.
Figure 3
