SD-04-400
Bendix® Reservoirs
DESCRIPTION

The reservoir is a storage tank; its function is to provide
a volume of compressed air for braking which will be
adequate in relation to the volume used by the brake
chambers and auxiliary devices. If the vehicle is not
equipped with an air dryer, reservoirs provide a location in
the system where the air, heated by compression, may be
cooled and the water vapor condensed.
Bendix® reservoirs are built in accordance with SAE
Surface Vehicle Standard SAE J10. They are made from
steel sheet, with stamped heads and rolled shells. The
seams are electrically welded, and each reservoir is
internally coated for corrosion resistance. Each reservoir
is tested at 300 psi hydrostatic pressure.
Reservoirs are supplied in various pipe ferrule arrangements
and lengths – and in diameters from 31/2-in. to 14-in. – having
various volumes from approximately 100 to 7600 cubic
inches for tractors and trailers. All ferrules are tapped to
SAE dry-seal pipe thread standards.
Reservoirs are also supplied in double- and triplecompartment configurations.

Figure 1 – Typical Single-Compartment Reservoir

PREVENTIVE MAINTENANCE
Draining air system reservoirs daily is discouraged when
powered vehicles are equipped with a properly functioning
desiccant-type air dryer. Daily draining tends to saturate
an air system (with a properly functioning air treatment
system) on initial charge, and it also causes the system’s
air compressor to work unnecessarily, thus reducing
its durability. Rather than fully draining a reservoir, it is
recommended to open the drain valves only long enough
to expel any moisture that may be present.

OPERATION

The reservoirs in an air brake system primarily serve to
store energy in the form of compressed air. They also
perform the less obvious function of providing a means
of cooling the air as delivered from the compressor, and
thereby condensing water vapor into a liquid, as well as
collecting oil passed by the compressor. This water and
oil collects as an emulsion; the greatest amount in the
reservoir nearest the compressor. It should be drained off
either manually or by means of an automatic drain device.

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INSTALLATION

Reservoirs should be securely mounted where they will be
protected against outside damage so they will not vibrate
or move during normal operation of the vehicle. A vibrating
reservoir usually causes broken tubing lines.
They should be the low point in the air brake system and all
lines connected to them should drain toward the reservoir.
The pipe tapped openings in the reservoir should not,
under any circumstances, be reduced in size from original
installation.
A drain cock or draining device must be installed in the
bottom connection of every reservoir and, in the case of the
two‑compartment reservoir, in each compartment.
The first reservoir or first compartment must be protected
by installing a safety valve.

MINOR REPAIRS

Minor repairs to the reservoir consist of examining the
reservoir mounting and the inspection of the outside for
corrosion or damage. The outside should be kept painted
to prevent the possibility of corrosion causing a failure.

MAJOR REPAIRS

Repairs involving welding should never be performed on
reservoirs. If a reservoir has been damaged and is unfit for
use, it should be replaced with a new one.
In exceptional cases where the inside of a reservoir has
become excessively coated with sludge which cannot be
drained off, it is sometimes advisable to remove it and
clean it with a solvent, steam, or water. If a solvent is used
to clean the reservoir, the reservoir should be thoroughly
aerated before reinstalling.

TECHNICAL SUPPORT
For direct telephone technical support, the Bendix
Te c h Te a m i s a v a i l a b l e a t 1 - 8 0 0 - A I R - B R A K E
(1-800-247-2725), option 2, Monday through Thursday,
8:00 a.m. to 6:00 p.m. and Friday, 8:00 a.m. to 5:00 p.m.
ET. Follow the instructions in the recorded message.
The Bendix Tech Team can also be reached by email at
techteam@bendix.com.

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SD-04-400 Rev 007 © 03/2026 Bendix Commercial Vehicle Systems LLC, a member of the Knorr-Bremse Group • All Rights Reserved