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Topic: Steps for purging air prior to replacing Air Dryer (Read 1145 times) previous topic - next topic

Steps for purging air prior to replacing Air Dryer

Have a 1998 U270 Unicoach and have decided to replace the Haldex air dryer. No record of service on it. I've owned the coach one year and put six thousand miles on it. I'm also going to replace the dash emergency brake switch that refused to release on last trip. Please fill in the blanks for me:

1. Block wheels
2. Raise coach and put eight safety stands (one at each air bag)
3. Dump air bags with safety stands supporting suspension
4. Open valve in front of driver rear wheel and bleed off air from wet tank
5. ????
6. Clean dirt off dryer and all fittings and photograph, label lines
7. Remove lines
8. Remove bolts from dryer mounting bracket
9. Purchase reman from NAPA
10. Purchase Ebrake switch for dash at NAPA
11. Purchase isolation valve attached to dryer if so equipped
12. Switch fittings to new dryer
13. Mount new dryer
14. Attach lines
15. Replace dash switch
16. Fire up engine and build pressure, listen for leaks
17. Raise suspension and remove stands

Should the aux compressor dryer be replaced? I don't know what it looks like or where to find it.
I only bleed air off at wet tank. Never see moisture. Are the other tanks equipped with bleed valves and should they be checked?

I have read the latest two threads on air dryers. That's why I'm going to replace my dryer, to get a baseline on it.

Re: Steps for purging air prior to replacing Air Dryer

Reply #1 –
That list looks pretty good to me.  I've replaced our dryer twice in our driveway.  You'll get hot and dirty, but it's not too bad a job.

Search the term "desiccant" on this forum for info on the aux compressor dryer and location.

All of your air tanks should have water drain valves.  It is a good idea to check them regularly, but it is not necessary to drain the front and rear tanks for this operation.

The blank line in your list (#5) doesn't need any entry.  When you bleed the pressure off the wet tank, that will relieve the pressure on the dryer and lines connected to it.  If the check valves at your front and rear tanks are good, you will retain the pressure in those tanks even when the wet tank is empty.  If the pressure in your front and/or rear tank drops to zero, then the respective tank inlet check valve is bad and need to be replaced.

While you're at the store, you might want to pick up the parts to make a Emergency Air Dryer Bypass manifold.  If you're not familiar with this term, see the thread linked below.

Some of the fittings on your dryer and air hoses may be pretty tight, so eat your Wheaties before you get started.  Good Luck!

Air Dryer Bypass Demo

Re: Steps for purging air prior to replacing Air Dryer

Reply #2 –
I am just a little concerned that you do not purchase the parts you need until steps 9, 10, and 11. You seem to be putting a lot of faith in your local NAPA store having all the parts in stock.
 
Good luck,
 
Trent

Re: Steps for purging air prior to replacing Air Dryer

Reply #3 –
There is a rebuild kit available for the "dash emergency brake switch" (Bendix PP-1).  Don't know how much luck you would have finding it locally, but might be worth a try:

Bendix PP-1 Repair Kit

Re: Steps for purging air prior to replacing Air Dryer

Reply #4 –
As long as you a digging this deep into your air system, you might as well take a look at your D-2 governor.  First, you must find it (not easy on some coaches).  Then visually assess the condition.  Is it fairly clean, or totally covered in grime, grease and corrosion?  If the latter, it might be time for a "preemptive" replacement.  Or at least buy a spare and carry it with you - cheap insurance.

See the thread linked below for a discussion of signs of failing D-2:

D2 governor identification

Re: Steps for purging air prior to replacing Air Dryer

Reply #5 –
Point taken on trip to Napa. Will the dash switch be under pressure? Don't want to disconnect and have a circus under there.

Re: Steps for purging air prior to replacing Air Dryer

Reply #6 –
As long as you are developing a really detailed list, don't forget to shut any tank drains you opened before firing up. Took me a second to figure out why I wasn't building up air pressure after having done some system repair.

Re: Steps for purging air prior to replacing Air Dryer

Reply #7 –
Quote from: Realmccoy –
Will the dash switch be under pressure?
Yes, sorry, I forgot about that.  :facepalm:  You will need to drain your front and rear air tanks to insure zero pressure at the PP-1.

Re: Steps for purging air prior to replacing Air Dryer

Reply #8 –
By the way, the dash parking brake valve comes in different pressure ratings.  The PP-1 type valve is designed to automatically set the rear spring brakes when the brake system air pressure drops below a set minimum pressure.  On our Foretravel coaches, this minimum pressure is around 35-40 psi.  If you buy a new control valve, be sure it "releases" at the correct pressure.

TYPES OF BRAKE SYSTEMS [ForeForums Foretravel Motorcoach Wiki]

Re: Steps for purging air prior to replacing Air Dryer

Reply #9 –
Quote from: TulsaTrent –
I am just a little concerned that you do not purchase the parts you need until steps 9, 10, and 11. You seem to be putting a lot of faith in your local NAPA store having all the parts in stock.


+1 - ours had to get the air dryer from a warehouse.

Also, you'll want to have the new air dryer on hand before you start removing the old one (or at least before you turn it in to get your core charge back) since you'll need to transfer some/most of the fittings.  You don't want to have accidentally turned those in with the core  :)

BTW it's handy to have a helper to hold the air dryer while you mount it.  I'm usually the designated "support staff" and it helps me understand the workings of the coach better.

Re: Steps for purging air prior to replacing Air Dryer

Reply #10 –
You can pump the brakes to get rid of the air. The slide pins can use the exercise anyway.
Break all the fittings loose before you you remove the dryer.

Re: Steps for purging air prior to replacing Air Dryer

Reply #11 –
Not trying to hijack the thread but let me make sure I followed this correctly.  First Air system work.

After draining pressure from rear driver side valve, I can release remaining air by depressing brake peddle till gauges read 0.  Correct?  I am replacing the brake switches.  I am not going under coach so I don't intend to use the stands.

Re: Steps for purging air prior to replacing Air Dryer

Reply #12 –
George,

Depressing (fanning) the brake pedal will work, but it's easier/quicker to simply open the condensate drain valve on each of the tanks.

(Don't forget to close them again when you are done with your project)

Re: Steps for purging air prior to replacing Air Dryer

Reply #13 –
Draining the wet tank is all you need to do to change the dryer.  The check valves on the other two tanks will maintain their pressure just fine without risk.
 

Re: Steps for purging air prior to replacing Air Dryer

Reply #14 –
I changed the air dryer on my 96 coach without drawing the air.  The check valve on the wet tank kept the air in the tank.