Vacuum Installation


Brake servos increase the force within the braking system by using vacuum applied to one side of a diaphragm. The diaphragm, being of a larger surface area than the piston in the brake master cylinder, increases the force transmitted to the wheel cylinders.

A crude example would be:

  • Without a servo, the relative size of the piston applying force to the wheel cylinders is about the size of a 1 CHF coin.
  • With a servo, the relative size of the piston applying force to the wheel cylinders is about the size of a side plate.

A very detailed description of the workings of the Servo I have fitted can be found on the MGB Organization website.

So where does the vacuum come from?

A vacuum can be created by removing air from a space using a vacuum pump or by reducing the pressure using a fast flow of fluid, as in Bernoulli’s principle.

Encyclopedia Britannica

For LGL, we’ll be using the Bernoullis approach… no moving parts.

It’s not uncommon to fit a belt driven vacuum pump to the engine and connect this to the servo. I have done this on a diesel Series 3 before but only because the vacuum take off on the old diesels is woefully inadequate. A pump from a Peugeot 205 works a treat along with a military lower double pulley to run the belt. You’ll have to fabricate something to fit it to the front of the engine block.

Fortunately there is a vacuum take off cast into the new intake manifold supplied by HNJ Engineering as part of the SU carb upgrade. On later models this was intended for recycling any gases that had blow past the valves. A vacuum could also be take from the main intake manifold. But as this is after the carb, there is a possibility you can draw petrol into the servo. A non return valve or a deep loop in the vacuum tube would solve this but both are quite intrusive.

Enough words… let’s look at some pictures.

The spigot supplied by HNJ Engineering screws into the intake. As air is drawn across the end, vacuum is created. The black hose connected to the spigot runs to the servo.
The vacuum hose terminates at the servo. Here the new brake lines can be seen.

The last job is to refit the wing…

Perfect fit!

It’s important to note that the introduction of a brake servo to a brake system in poor condition, will do little to improve it. There should be no leaks and brake pads adjusted correctly and service operations be carried out as recommended.

I’ve yet to test drive the vehicle with the new servo as I need to address an issue with the leaf springs. More on that in the next post.