Brake Switches

Figure 1


Problems with the original brake switches

Stop Lamp Switch

The hydraulic stop lamp switch, Lucas part SPB401, can be prone to fail and some owners fit a microswitch operated by the brake pedal. My original Lucas switch lasted years, but since the rebuild, I have replaced the switch twice. They are prone to fail under light braking. An article "Hydraulic Brake Light Switch Failure Investigation by R.Kwas" was posted on the internet in 2007 and is really worth reading. Replacement switches are available on the internet for less than ten pounds, but always check the thread size as they vary. On my Morgan the Lucas part SPB401, Intermotor 51610 or Unipart DAC3719 with 3/8" x 24 UNF thread size is correct. They are available from Auto Electrical Spares. Before any journey I always check the rear brake lights are working properly. This involves switching on the ignition and lightly depressing the brake pedal to check whether the rear brake lights illuminate. If they only come on when putting excessive pressure on the brake pedal, the hydraulic stop lamp switch needs replacing.

Brake Warning Lamp Switch

The original master cylinder on my 1972 Morgan did not incorporate a brake failure warning light. This function was provided by a differential brake valve differential brake valve bolted to the offside chassis frame. The main body of the valve contains a floating piston, which is positioned between the rear and front brake hydraulic circuits. If pressure drops in either the front or rear circuit, the piston moves towards the lower pressure one and isolates it. The piston also activates the warning light switch. The healthy side of the brake hydraulic circuit maintains its integrity, whilst the warning light informs the driver of reduced braking ability. When I phoned the Morgan Motor Company for spares to service the master cylinder and differential valve, I was advised to scrap the old valve and cylinder. They sold me a new dual circuit master cylinder with a built in warning switch. Although original components are important during rebuilds, Morgans advice was sensible. Not only is the modern master cylinder better, it makes brake servicing a lot easier. It was a fiddly and time consuming business if the floating piston in the differential valve moved when bleeding or replacing brake components (see Ford Capri Mk1 Haynes manual for details). If the dual master cylinder with built in brake warning switch is fitted, the hydraulic pipe runs become less complicated, as there are fewer pipe unions and a reduced potential for leaks.

Brake Warning Lamp

When the ignition is switched on, the brake warning lamp will glow if the fluid level in the master cylinder drops below a safe level. It will also be illuminated when the handbrake is engaged. If applying the handbrake fails to illuminate the lamp, remove the dual master cylinder lid. Should the lamp still refuse to glow, replace the bulb.

If the brake warning lamp fails after performing the above checks, it is most likely because of a wiring or fuse fault. The ignition auxiliary fuse supplies power to the brake warning light when the ignition is switched on, whilst the handbrake and master cylinder switches complete the circuit to earth as shown in Figure 2. The fuse can be checked by using the indicators or switching on any other accessory listed under Ignition auxiliaries. If the fuse is OK and the brake warning light is still refusing to work, the fault will be due to the wiring. Check the snap and Lucar connectors are tight and corrosion free before replacing any faulty wires. Also check the earth connections and the handbrake micro switch.

Figure 2

Morgan Brake Warning light

When reassembling the brake hydraulics, DO NOT be tempted to split the dual braking system diagonally. Suspension systems on modern cars are designed to work with the diagonal system, whilst Morgans with sliding pillar suspension are not (see Brake Hydraulic Layout).