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Air Tank Valves R&R

March 16, 2016, 11:51:54 am

I am starting the next step in my Air System Renewal Project.  I plan to replace the control valves mounted on the Front and the Rear air tanks.  My Front and Rear air tanks are both mounted at the front end of the coach, directly behind the fiberglass "Bumper".  This may not be the case for other year/model coaches, but the control devices associated with each tank should be similar (but not necessarily identical).

This should be a fairly simple "remove and replace" exercise.  I am documenting it for my own benefit, so I can remember what I did, and when I did it.  Plus, my narrative may encourage others to take a more active interest in the workings of their air system.

Before you start tearing things apart, it is worthwhile to spend some studying the layout of the hoses and valves around the air tanks, and comparing them to your Air System diagram.  The factory diagram is helpful in tracing air line routing, but it does not always show the actual physical location and/or connection of hoses and valves.  I had to make several sketches of my various tank connections, then I spent considerable time comparing them to the Air System diagram, before I really understood how it all worked.

My first task will be to replace the Protection Valve on each tank.  See next post below.

Photos below:  The front air tank, with original Bendix PR2 protection valve (the one with the safety wire securing the adjustment cap)

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Re: Air Tank Valves R&R

Reply #1 – March 16, 2016, 11:54:15 am

The Protection Valve.

There is one on the Front tank, and One on the Rear tank.  The purpose of this valve is to retain a minimum quantity of air in the tank, in the event of a catastrophic air loss downstream of the valve.  If the Front and Rear air tanks were only used to operate the brake system, this valve would not be necessary.  However, on our coaches, these tanks are "multi-purpose".  They not only provide air pressure to the brake system, but also to the suspension system, the throttle/cruise control (on some models), the air step/cover, etc.  If a hose were to be cut...say to one of the air bags, then the pressure in the respective air tank could be rapidly depleted.  A properly functioning Protection Valve would close, retaining enough pressure in the tank (usually about 65 psi) to operate the brakes and bring the coach to a safe stop.

The OEM valves on my coach were made by Bendix, Type PR2, part # 279505N.  This same part is still available.

279505N by BENDIX - VALVE PRESSURE PROTECTION

Photos below of one of my old valves dissected.  The "supply" air comes in the side port, where it is trapped between the small round black seal and the white "piston".  When supply air pressure is high enough, it will overcome the large black spring, and force the white piston to rise.  This movement raises the small black seal off it's seat, and allows air to flow out the "delivery" port at the bottom of the valve.  As long as "supply" pressure is greater than the (black spring controlled) pressure setting, air will flow.  If "supply" air pressure drops below the pressure setting of the valve, it will close, trapping the remaining air in the tank.  On our coaches, this OEM valve is set at 65 psi, and safety-wired to prevent tampering.

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Re: Air Tank Valves R&R

Reply #2 – March 16, 2016, 12:30:33 pm

New Protection Valve installed on tank.

My next post will cover the other new valve (attached to the Protection Valve), which is called a "Shuttle Type Two-Way Check Valve".

You may notice that I rearranged the positioning of the valves on the tank.  I will explain my reasoning for this in my next post.

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Re: Air Tank Valves R&R

Reply #3 – March 16, 2016, 12:39:07 pm

Chuck,
Another great topic of general interest to practically all Foretravel owners.... Keep em' coming!
Don

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Re: Air Tank Valves R&R

Reply #4 – March 18, 2016, 06:58:54 pm

The second valve I found on my "Front" and my "Rear" air tanks is called a "Shuttle Type Two-Way Check Valve".  On my coach, it is connected directly to the "Protection Valve".  It has 4 openings - two marked "supply" and two marked "delivery".  There is a threaded cap on one end, which when removed gives access to the innards of the valve.  Inside there is a simple barrel shaped sliding valve, with a rubber sealing surface on each end.  There are no springs.  See photos below.

How it works.  The purpose of this valve is twofold.  First, it allows two separate "supply" sources to provide pressure to the 2 "delivery" ports.  If we call the "big" end of the valve (with the removable cap) the primary supply port, then the opposite end of the valve is the secondary supply port.  The sliding valve can seal off either supply port (but not both), depending on which way it moves.  It will always try to move toward, and seal off, the supply port with the LOWER pressure.  If both supply ports are at the same pressure, the sliding valve will be inoperative - i.e. it won't do anything.

Suppose the pressure in one of the supply ports suddenly dropped, due to a broken air line.  Then the valve would serve its second purpose: a safety check valve.  It would immediately slide toward the supply port that was losing pressure, and seal it off.

The OEM valves on my coach were manufactured by MIDLAND, part # N20966M.  The current (identical) replacement part is made by HALDEX, part # KN25120.

KN25120 by HALDEX - Shuttle Type Two-Way Check Valve

If you go to the linked page, you will notice that there is a very clear statement: "Only mount horizontally as pictured."  There was also a little sticker on each new valve, saying the same thing.  The valve is designed to work best when mounted horizontally.  Which brings us to a curious finding, at least on my coach: both valves were originally mounted vertically.  This caused both of my valves to be non-functional, due to a buildup of trash and greasy residue in the supply port that was facing (incorrectly) downward.

It is hard to see in my last photo, but the metal surface where the rubber seal rests was covered with trash.  The valve could not seal off this supply port, so it could not serve either of its functions.  I believe this problem can be avoided by mounting the valve in a horizontal position, so that is what I elected to do.  It was not difficult to rearrange the valves and fittings into the new alignment.  In the photos below, you can see the valves on both tanks are now mounted "correctly", according to the manufacturer's instructions.  A secondary benefit is that the valves are now mounted higher (above the ground) and are less susceptible to damage from road gators.

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