Negative pressure hose: take advantage of the power of atmospheric pressure

The normal atmospheric pressure near the ground is 1 bar. This natural air pressure is normally not even felt. But when it is combined with a generated vacuum, however, it can be used for many things. And that’s where the negative pressure hose comes in!

Negative pressure on cars

There are several applications for negative pressure or vacuum on cars.

The best-known application is the brake booster. This is a large black box that is attached directly to the bulkhead behind the steering wheel.

The brake master cylinder or brake pressure distributor is attached to the brake booster. When stepping on the brake pedal, the force from the driver’s leg is not enough for comfortable braking. Therefore, a vacuum is created in the brake booster. As soon as the driver steps on the brake, the natural air pressure additionally presses on the brake master cylinder and thus amplifies the pedal force of the driver.

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Other applications for vacuum are:

– Turbochargers – Pneumatic central locking – Direct injection for diesel and petrol engines

How is the negative pressure generated?

Over the decades, various systems have been used to generate vacuum in the car. They range from secondary lines connected to the exhaust system to separate vacuum pumps.

• Today, these are still used as standard in trucks, for example.
• In passenger cars, they were still installed in French Peugeot diesels until the 1990s.
• In cars today, however, secondary lines on the intake manifold are favoured for cost reasons.

Challenge of the negative pressure hose

To use the negative pressure at the desired location, it should be diverted from the point of its output. This is where the vacuum hose comes into play. Although a vacuum hose looks like a normal rubber hose, it is nevertheless a highly specialised component that serves to cope with many challenges.

A vacuum hose should be

– Rigid
– Oil and petrol resistant
– Airtight
– Heat-resistant
– And durable

• Rigidity is probably the most important property that a vacuum hose should have. If a vacuum hose is replaced by a normal, soft rubber hose, this usually has nasty consequences: The vacuum causes the hose to contract.
• Then the hose closes the vacuum line like a membrane. When the vacuum is then opened, it can no longer be built up. Depending on what the vacuum was used for, this has fatal consequences: In the worst case, the brake fails.
• To ensure that the negative pressure hose lasts as long as possible, it should be made of very resistant material. This is not so easy with rubber, which is usually made with solvents as plasticisers.
• The highly oily and petrol-laden atmosphere in an engine compartment therefore permanently damages the vacuum hose. So, it should be heat-resistant and fireproof to withstand the high temperatures in the engine compartment.
• The airtightness of a vacuum hose is the most important property, along with its rigidity. If the negative pressure hose draws air, no vacuum can be created, and the assembly will fail. This is one of the typical defects of the vacuum hose.

Defects in the vacuum hose

A negative pressure hose may actually have only one defect: it will leak. With a bit of luck, it is only a little loose in the connection area. Then it is sufficient to tighten the clamp to restore the tightness.

However, due to the high stresses or mechanical influences, the vacuum hose can also get a hole. This happens gradually due to petrol or oil in the ambient air. Another common case is a marten bite or when the vacuum hose rubs against a belt or pulley.

You can notice a defective vacuum hose quite quickly: there is an audible whistling and hissing noise in the engine compartment and the affected assembly has reduced performance or fails completely. For example, if braking is very sluggish despite the engine running, a porous vacuum hose may be the cause.

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Repairing vacuum hoses

Fortunately, repairing a negative pressure hose is one of the easiest car repairs.

• They are connected to two connectors of the air line with hose clamps. These are simply loosened with a screwdriver and the vacuum hose can be removed. On some vehicles, the clips should be loosened, but this is usually self-explanatory.
• The most important thing when repairing or replacing negative pressure hoses is to avoid DIYs. You can seal the hose with insulating tape for a short time. However, this repair should only be made until the next workshop. A defective vacuum hose should be replaced – and then thoroughly inspected.
• If the negative pressure hose has needle-sized perforations, then it could be a marten bite. This is especially true if the holes are on the opposite side of the hose. A marten bite is typically characterised by a bite-counter-bite pattern. In this case, all other lines in the engine compartment should be thoroughly examined.

These include:

– Power cables – Water lines – Belts – Ignition cables – Housings

Anything soft enough can be nibbled.

In this case, it is time to take countermeasures! These include a thorough engine wash and the installation of an ultrasonic marten deterrent.

However, if the negative pressure hose practically disintegrates in your hand, then it has reached the end of its service life. In that case, all other vacuum hoses in the engine compartment should be examined. They will probably show a similar picture.

In the case of an obvious abrasion point, the negative pressure hose has come loose and come into contact with a rotating component. In this case, it is sufficient to replace the affected hose and pay close attention to laying it correctly.

Pay attention to quality

Depending on the application, vacuum hoses are highly specialised or manufactured as piece goods. The precisely manufactured vacuum hoses are usually many times more expensive than the universal hoses.

On the other hand, they are stiff and very durable. The flexible hoses made by the metre tend to come loose and then get damaged on rotating components. Therefore, our recommendation is to always install the appropriately designed hose. This guarantees a durable and safe solution.

It is important to pay attention to brand quality. You should be suspicious of all too cheap offers. You can’t tell from the rubber how good its compound is.

Only the formula specified by the car manufacturer should be used for the production of vacuum hoses. Car manufacturers issue licences and certificates for this.

It does not always have to be the original equipment quality. However, a reputable, traditional brand for car spare parts should always be preferred as the supplier for the new vacuum hose.

Foto: Standret, Wachira W, GrooTrai, Sharomka, pixinoo, MoonAngelEyes, Leove / shutterstock.com