Air Valves Compared: What are the Alternatives
A truck’s air system can seem complicated and, considering the difficulty of diagnosing braking problems, some of us resist repairing a system with components from different suppliers. Yet, there are opportunities to substitute when individual valves are exact equivalents. These opportunities exist more often with the simpler, basic valves. Obviously, it is also not necessary that trailer valves come from the same manufacturer as the tractor valves. Therefore it helps to know all of the players.
Bendix, Haldex, and Sealco Valves Compared on following pages.
Pipe Thread TerminologyBendix most often specifies the port sizes of air valves and other system components with the abbreviation “PT”. Haldex, Sealco, and other companies supplying air hose fittings prefer to use the abbreviation “NPT”. “PT” is understood by everyone to mean “pipe thread” and is the Bendix way of representing “NPT” the official standard engineering abbreviation for “American Standard Taper Pipe Threads”.
With air valves you may also see the abbreviations “PTF” and “NPTF” which was an enhancement of the “NPT” standard to accommodate the more demanding fuel applications. In any case, PT, NPT, and NPTF are all compatible threads. |
Importance of Maintaining Orientation of Ports and Mounting Configuration One stumbling block that you might not expect when replacing air valves is the requirement that the ports face the same direction when the valve is attached to the truck or another component of the air system. The balance of the air pressure in the system can be affected when you do something as simple as add an extra ninety-degree elbow fitting in order to connect an existing air line to a new valve.
A ninety degree turn adds friction to the air flow and it has been estimated that one turn is equivalent to adding a delay equivalent to six feet of air line. This is not to say that a replacement valve cannot be mounted with a different orientation but only that the effect on system balance must be considered. |
Materials from which air valves are made
The choice between a metal or a composite valve is one of the first alternatives you may have in choosing a replacement valve. Traditional air valves used in trucks can best be understood as a variation of brass metal consisting of copper, zinc, tin, and lead. It is a non-ferrous alloy that will not rust and is easy to machine.
Composite materials are often given the disparaging label of “plastic”. This is only partially true. An “engineering grade” composite falls into a narrow range of polymers known as thermoset plastics which can have even greater hardness than metals. In any case, you can count on the fact that this material will not melt when subjected to heat. |
Composite materials have a bright future as truck components because they can be produced in one operation with no machining required resulting in lower production costs. And of course they weigh less.
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