Airpack Questions

[houdel]

I have a couple of general questions concerning the air-hydraulic unit used on the Deuce. I have searched the 209 and 361 series -20 through -34P manuals and cannot find much info on the AHU.

First, does anyone have an exploded view of the AHU? the -34P TM only shows the AHU as a single part and does not give a breakdown, so apparently they were not serviced at the DSU/GSU level. But I have seen pictures of rebuild kits with gaskets etc. so they must have been rebuilt someplace.

Second, can someone explain how the AHU functions to "power assist" the breaking system? I am familiar with typical automotive vacuum/hydraulic systems, but on those systems assist unit is located BETWEEN the brake pedal and master cylinder. The brake push rod acts on a diaphragm or valve in the assist unit and the assist unit "powers" the push rod to increase the force applied to the master cylinder by the push rod. In event of an assist system failure, the push rod still directly activates the master cylinder but all the braking force is provided by the operator's foot.

In the Deuce, the AHU is located AFTER the master cylinder, BETWEEN the master cylinder and the wheel cylinders. So how does the AHU increase the brake system pressure after the master cylinder, and how does the master cylinder bypass the AHU to activate the brakes when the AHU is not working or does not have sufficient air pressure to provide assist?

 

[cranetruck]

Lee, the Airpak has a "slave" hydraulic cylinder built-in. The slave cylinder is operated by the large air piston OR (no air pressure) by the Master Cylinder alone. The piston within the slave cylinder has a valve so it can operate by itself or with the air assist.
I have exploded views of the Airpak and will dig'm up later.

 

[Boatcarpenter]

Lee, it's been a while since I had mine apart and sort of figured out what it did, but basically, through the action of the big air operated cylinder, the push rod with cups etc. on the brake fluid side(front of the airpack), it has a great multiplying effect on the pressure to the brake cylinders. I think Bjorn has a guage (surprise, surprise) in one of his brake lines that monitors his system pressure. IIRC, the pressure is boosted from 400PSI to 1600PSI or something like that. Somehow it is valved so that if you loose air press., the pedal pressure actuates the brakes, the fluid has a clear path through the front plumbing, valves, etc. in the front of the airpack. In effect, as I recall, the boosted pressure comes into the system as though it were an entrance ramp onto a main highway. That way, with no air, you still have brakes.
Help any? It has been a while, but thats what I seem to remember figuring out.
Others have a far better understanding than I do I'm sure.
BC

 

[Boatcarpenter]

Bjorn will have you well acquainted with the airpack soon, I'm sure!!

 

[cranetruck]

Brake line pressure gauge? Me? Okay, yes, and the pressure without air is about 400psi and 1,800 with air.

Here is an exploded view.

 

[houdel]

Thanks Bjorn and Boatcarpenter. I suspected something along the lines of a secondary brake apply cylinder with air assist actuated by fluid from the master cylinder.

Bjorn - Where did you find the exploded pic of the AHU? Is there an electronic manual somewhere that I can download?

 

[OPCOM]

Bjorn, so the 400PSI is the about max no matter how hard you push on the brake pedal without air (to the floor?)

This is why the pedal firms up with increasing air pressure? I always wondered about this since noticing there are still a little brakes with no air.

[- unlike a vacuum assist on a car, where instead of 30 LBS on the pedal you can push with 300lbs if vacuum is lost to achieve the same effect.]

 

[cranetruck]

Patrick, just went out in the cold (25°F) and stepped on the brake, it registered a little over 500 psi. Sorry about the image being out of focus, I'll get it replaced another day.

Lee, the exploded view comes from an old Memphis Equipment catalog, don't know which TM they got it from.

 

[tjcouch]

Oooooooo . . . I want, er, NEED one of those line pressure gauge set-up!

Did you do a write up?