Those brake chambers are called a 30-30 can in the maintenance world. It's a spring brake and a service brake all in one. One hose is the air supply to release the emergency spring, the other hose is the service supply air. The service supply air is what actuated the brake chamber via the brake pedal. You'll have two air valves inside the cab. A red valve button and a yellow valve button. Yellow is to release the emergency spring in the brake chamber. It's what sets the brake when you park, OR sets the brakes in an emergency such as an air hose blowing cause you to lose air, in which case the emergency brakes will set automatically once it reaches 25psi.

The red air valve button supplies the trailer with air.

Yep, was just out running some errands and got behind a Mack dump truck with the exact same Haldex brake actuators and twin air lines running to them!

If interested; there is an explanatory video on basic truck air brake and spring emergency brake operation on You Tube at:

https://www.youtube.com/watch?v=QVJSFlp8Pws

HEMTT gun ring kit for sale on eBay! (Of course it's in 1 to 1 scale!)

Brake actuators, mounting brackets, slack adjusters
and S-cam rods ready to paint and drill!

Babe's got Brakes!

All four brake cylinders installed but nothing piped yet.

Looks great.

Thanks Dustin - glad you approve. Also thanks for that explanation on the double acting rear brakes.

You probably figured out that I have always been the kind of guy who wants to know "what's on the inside". Also I figure knowing how things are supposed to work can sometimes make for a better, more correct model.

Well I have a week's reprieve in that I just discovered that I had the dates for the Dayton IPMS competition wrong. It is the 17th of this month rather than this Saturday as I had thought. Perhaps I can get the gun ring done as well.

Dustin, the air for the rear brakes seems to go through some sort of distribution manifold/valve/controller. There is some confusion (on my part) reading the TM. Is there one distribution valve for each rear axle or is one valve for the service brakes and the other for the parking brakes?

These valves are mounted, one in front of and one just behind the rear H-frame member.

Can you post a picture of the TM?

Try'n to find it again!

Generally speaking, the air compressor will feed the air tanks, and then the air tanks will feed the distribution blocks and then they'll go to the brake chambers AND air bags if the application applies.

Here they are "Relay Valve". They look to be 4-6 inches square and roughly 3 inches tall.

Correct. One relay valve can supply multiple axles depending on how many ports it has. They also have a pressure relief clave built in.

Looks like there is one relay valve per axle and that both the service brakes and the parking brakes run thru the same relay valve.




Both these drawings had the identical more or less meaningless title. Also there was nothing to differentiate what one diagram illustrates as opposed to the other.

Great diagram there.

Mike, I was going to try a scratch those relay valves for my M983 but was scared away from it. Can't wait to see how you tackle them.

Hi Mike

really coming along nicely, and thanks for sharing all the diagrams as its really helping me in quest to build multiple HEMMTs at the moment

Keith

ALL of you guys are providing and sharing EXCELLENT info, drawings, etc! It's a welcome change from some of the other threads on this site, where people are bickering over some of the most INCONSEQUENTIAL CRAP, the "put-downs", and just out-and-out peevishness...

I've been following this thread right along from it's inception, and I've learned A LOT about the complexities and details of modern US Army vehicles!

THANKS to ALL!!!

Thanks Dennis, WOW!

Building the Relay Valves:

Start with some 3/32" tube and add four pieces of .035" rod.





Glue assemblies to some thin plastic sheet. (True up the vertical rods and allow to dry thoroughly.) Then I used some .080" octagonal rod to control the height of my sprue cutters in order to trim all the rods to the same .080" height.

This will be THE go to thread for HEMTT builds! Nice work, Mike!

I do truly apologize for not announcing the following the minute I discovered it!

On the Italeri model (I can't speak for the Trumpeter kit.) the front four wheel drums are different than the rear four. Special care should be taken to insure that the correct drums are used in the correct locations. However there is nothing about the model that prevents you from putting the wheel drums on incorrectly! The most visible difference between the front and rear wheedl drums is that the wheel center housing projects out further on the rear wheel drums than it does on the front four drums.

As with most REAL wheel drums, the center of the drum is off-set relative to the centerline of the tire tread. With the HEMTT the off-set wheel drum can be mounted on the vehicle facing either way. Due to the presence of the steering mechanism on the first two axles the off-set of the wheel drum is to the outside to allow more space behind the wheel.

On the rear two axles the axles are actually narrower (no steering mechanism) so the off-set of the wheel drum faces the inside. Doing this keeps the finished tread width of all four axles approximately the same.

The same is true for almost any heavy truck.

As per the Italeri instruction sheet the front wheel drums are identified as parts 2G. The rear wheel drums are identified as parts 1G. Finally the wheel drum for the spare is identified as part 3G and has no wheel center at all.


. . . . . . . . . . . FRONT WHEEL DRUM . . . . . . . . . . . . . . . . . . . . . . . . . REAR WHEEL DRUM . . . . . . . . . .
. . . . . . . . . OUTWARD FACING OFF-SET . . . . . . . . . . . . . . . . . . . . . INWARD FACING OFF-SET . . . . . . . .
. . . . . . . . . . . LOW WHEEL CENTER . . . . . . . . . . . . . . . . . . . . . . . . . HIGH WHEEL CENTER . . . . . . . . .

More work on the Relay Valves:




A .035" disc in the middle for the air fitting and fine strip stock (sorry don't know actual size) for the X bracing. When dry trim X to final length.