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2026
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Can't tell if 'air topping oil' refers to the clutch booster or the braking system?
Air brakes and hydraulic brakes are no longer unfamiliar to us.
There is also a type of brake system called air-over-hydraulic,
which was commonly seen on Japanese trucks years ago.
Air-over-hydraulic brakes are also called air-actuated hydraulic brakes or air-to-liquid braking systems. They incorporate both pneumatic and hydraulic mechanisms.
The brake air chamber and the hydraulic master cylinder are connected in series. By converting pneumatic energy into hydraulic energy, the brake fluid is transmitted to each wheel cylinder, thereby producing braking force.
Compared with pure air brakes, the brake chamber volume in the air-over-hydraulic system is smaller and its layout is relatively compact.
In addition, the air-over-hydraulic brake system has these advantages:
1. The air pressure system has shorter pipe lengths, which can reduce lag time;
2. The hydraulic wheel cylinder, acting as an actuator, is lightweight;
3. When towing trailers, the trailer can choose either air brakes or hydraulic brakes.
Air-over-oil brake system
According to some veteran vehicle owners, the air-over-oil brakes indeed provide good braking force and respond quickly. They are not as sudden as the braking force of air brakes, but they also have disadvantages.
High temperatures during long downhill drives can deteriorate the performance of brake fluid, and the seals (cup seals) of the hydraulic wheel cylinders (slave cylinders) can melt or even fall off, causing brake fluid leakage and a sudden loss of braking force.
In addition, long-term maintenance and disassembling of brake drums are particularly troublesome.
In terms of reliability, it is ultimately inferior to the relatively simple structure of pure air brakes.
Today, except in loaders, dump trucks, and other vehicles requiring a hydraulic system, it is already very difficult to find air-over-hydraulic brake systems.
In addition, a type of clutch booster on trucks is often referred to as 'air-top-oil.'
In fact, this expression is inaccurate.
Clutch Booster
Some trucks, especially medium and heavy trucks, have relatively high clutch pressing force.
To reduce the effort required for the driver to press the clutch pedal,
a clutch booster, also called a clutch servo or booster pump, is usually used.
This device uses both a cylinder and an oil cylinder,
so vehicle enthusiasts may think it is similar to the principle of air-top-oil brakes,
but in fact, the principles of the two mechanisms are quite different.
The clutch mechanism uses brake fluid as a medium, starting from the clutch pedal,
and ultimately transmits the force to the clutch fork, acting on the release bearing.
Pressing down the pressure plate diaphragm separates the pressure plate from the clutch plate, interrupting the transmission.
Generally, non-assisted hydraulic clutch system:
Pedal — Clutch Master Cylinder (Hydraulic) — Oil Pipe —
Clutch Slave Cylinder (Hydraulic) — Clutch Fork
Clutch system with assist cylinder:
Pedal — Clutch Master Cylinder (Hydraulic) — Oil Pipe —
Clutch Assist Cylinder (Pneumatic-Hydraulic Integrated) — Clutch Fork
When installers add a clutch assist pump to some light trucks,
two issues need attention:
First, there must be an air storage tank (mandatory for air-brake vehicles),
second, there must be enough installation space.
On the market, there are clutch assist pumps with cylinder diameters ranging from 70mm to 120mm,
and light trucks generally only need the small-diameter ones.
Some might say,
wouldn’t it be better to use air pressure directly for the clutch?
But from the experience of using air brakes,
air pressure controllability is not very good,
and the clutch requires semi-engagement control,
so using pure air pressure is not very practical.
In summary,
air-over-hydraulic brakes use an air chamber in series with a hydraulic cylinder,
with braking executed hydraulically,
while a clutch assist cylinder is pneumatic-hydraulic integrated,
using air pressure to provide assistance.
From this perspective, clutch assistance cannot really be called air-over-hydraulic.
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