tube and jar is recommended regardless of the method
Most authorities recommend that the system
should be bled starting with the longest line and
used to force fluid through the lines.
working successively to the shortest. Other authorities
You can use a manual method that requires two
prefer the opposite method; that is, starting with the
persons. One person operates the brake pedal,
shortest line and finishing with the longest. So, you
pumping until the pedal action is hard--hydraulic
should always consult the technical manual for the unit
pressure in the system. The other person opens the
you are servicing to see if there is a preferred method.
bleeder valve screw, allowing the air and fluid to
If you suspect that air is trapped in one specific
escape as the brake pedal is still forced downward. As
brake line, bleed that line first. For example, assume
soon as the brake pedal nears bottom, the operator
that the right rear wheel cylinder of a vehicle has been
signals the other person so that person can close the
disconnected for maintenance and repair. After the
bleeder valve. As soon as the valve is closed, the brake
repair has been completed and the cylinder connected,
line pressure is again pumped up and the bleeder valve
this brake line should be bled first. In fact, it is possible
opened, repeating the process until the air is expelled.
that all of the air that entered the line while it was
When bleeding a hydraulic brake system that
disconnected may be bled from the one bleeder valve.
consists of two wheel cylinders, an upper and a lower
If at all possible, use a length of flexible tubing and
on each wheel, the operation is slightly different. In
a clean glass bottle or jar to trap the brake fluid
that case, bleed the upper cylinder at each wheel first,
expelled from the wheel cylinder bleeder valve. Before
and then bleed the lower cylinder.
attaching the tube, be sure to clean the bleeder valve
end.
High-Pressure Hydraulic Brake Systems
To bleed the line, first loosen the bleeder valve
The latest type of hydraulic brake system is the
screw. This allows fluid to flow into the jar. Keep the
s t y l e u s e d o n t h e a i r c r a f t t ow t r a c t o r, M o d e l
end of the tube submerged in the fluid. By observing
A/S32A-37 (fig. 2-41). This system has a brake pedal
the flow from the tube, you will notice air bubbles as
(called a brake actuator in this equipment), but not a
they appear. When the air bubbles stop, all the air has
master cylinder. In place of the master cylinder, there is
been expelled from that section of line and the wheel
a spool valve in the actuator that controls the flow of
cylinder. Then, tighten the bleeder valve screw. Repeat
this procedure with each bleeder valve. The use of the
hydraulic fluid to apply pressure on the disc brakes.
PUMP
FLUID
RESERVOIR
ACTUATOR
ACCUMULATOR
ASf02041
Figure 2-41.--Hydraulic system of the A/S32A-37 tow tractor.
2-33
