Hello, Matt here from Educational Mechanics.
Welcome back.
So, we know that the brake booster also known as vacuum servo, boosts the driver’s efforts at the brake pedal.
But there are a couple of common questions that we have been getting recently.
The first one.
I’ve got a turbo fitted, so where does the vacuum come from?
An question 2,
The brakes are boosted, but by how much?
So for starters, let’s have a look at the system components and layout.
We know we need a source of vacuum or negative pressure,
and in normally aspirated engines, this can come from the inlet manifold,
as when the engine is running it sucks air into the chambers on the inlet stroke
creating a negative pressure in the manifold.
On forced air induction, such as turbo or super charged engines,
there is an over pressure in the manifold,
so we can’t get the vacuum from there.
What we need to do is fit a vacuum pump somewhere.
Some engines have a direct drive engine pump which can be off the cam shaft,have
some a belt driven pump or even mount them to the back of the alternator,
and some can have electrically driven vacuum pumps which can be positioned pretty much anywhere.
In any case, it don’t really matter where your vacuum pump is driven from,
only that you have something that can create the vacuum.
Next in the system we have the servo unit incorporating the non-return valve,
we have other videos describing how the servo works which you can see by clicking here.
But in short it helps the driver and boosts their efforts.
The brake pedal is physically connected to the master cylinder through the booster unit,
and the vacuum is just there to help.
If it has no vacuum or the servo unit has failed,
the driver still controls the brakes, just not as effective.
So this leads us on to the second question,
How much are the brakes boosted by?
Well this can vary between manufacturers,
but what we have here on our training rig is the master cylinder and servo unit from a Land Rover.
We’re going to do a couple of tests
and look at the brake pressures while operating the pedal at a constant force.
The first test we are going to do is with the engine switched off so no vacuum for assistance.
Hmm, right, what we got there, with 20lbs on the brake pedal was about 250 psi brake pressure.
Right then, let’s start the engine and get us some vacuum.
Now with the same 20 lbs on the pedal
we now have almost 500 psi brake pressure,
that’s almost double the force available to us than without the vacuum.
Or if we look at it another way,
if our vacuum brake booster wasn’t working,
we would only have half the normal braking force available.
Now, as I said earlier, the degree of assistance will vary between manufacturer,
but it’s safe to say that the larger the diameter of the servo unit,
the more assistance we are going to get.
So, there we go then,
what I’d like you to do now is go and have a feel of your own brake pedal,
and test your own brake booster servo unit is working.
If you click up here, this video will show you exactly how to do these tests.
Thanks for watching.