Can you mix 'n match airpacks? - One long one short on an USAF Deuce.

Hey All,

I have a dual circuit brake deuce. '88 USAF model.

On of my airpacks has the rear mounting stud broken off (both are long style).
My plan has been to go thru the brake system rebuilding or replacing as I go, this just accelerates part of the plan.


So, I'd prefer to have short airpacks because of their design, but because of price and availability, it will be difficult to replace both at once.
Would there be any issues if I replaced one of the long airpacks (the broken one) with a short one?

Said differently, can you run one long airpack and one short one on a dual circuit deuce?


Thanks

ciao
lino

I don't see why running 2 different air packs will be a problem. This might cause a small pressure difference between front and rear but I don't see that being a serious issue.

They do not separately control "front and rear".

Best to state what you know, and not speculate.

doghead said:

They do not separately control "front and rear".

Best to state what you know, and not speculate.

Click to expand...

I'm a little confused by your statement. Am I incorrect that one air pack boosts the front brake circuit and the other air pack boosts the rear circuit?

Yes, you are.

A look at the TM will help you better understand it. Look at the parts TM for the brake lines.

Ah. Thank you for that correction.

welldigger said:

I'm a little confused by your statement. Am I incorrect that one air pack boosts the front brake circuit and the other air pack boosts the rear circuit?

Click to expand...

What connects to what?
View attachment SMARPI9-2320-209-14P.pdf

I had been digging thru that TM before I posted and found nothing that related to my question. Seems like it might b easier just to trace the brake lines under the vehicle.

ciao
lino

How would tracing the brake lines help you answer if you can use one of each air pack?

doghead said:

They do not separately control "front and rear".

Best to state what you know, and not speculate.

Click to expand...

welldigger said:

I'm a little confused by your statement. Am I incorrect that one air pack boosts the front brake circuit and the other air pack boosts the rear circuit?

Click to expand...

welldigger said:

Ah. Thank you for that correction.

Click to expand...

frank8003 said:

What connects to what?
View attachment 540252

Click to expand...

According to that manual, the dual circuit trucks do break the brakes up into front/rear split.



I tried tracing the lines on the image contained in the SMARPI supplement but I'll be darned if I can tell (I would have broken that into 2 separate images, one for hydraulic and one for air if I had been doing it) where what goes.



I'd go crawl under my '87 in the driveway if it wasn't pouring cats and dogs right now.

After studying the manual best I can tell is the system does split front and rear. The only place they tie together is the differential valve.

Well, it's not abundantly clear in the TM, but if front and rear were on different air packs then one might begin to argue that it's ok to have different air packs. If they do provide different amounts of assist, the effect would be a shift in front/rear bias. These aren't race cars, so a moderate change in bias might not be a big deal.

Most cars split circuits is a cross pattern. I'm pretty sure I read that these split front rear.

Another note is that the A2 with split brakes has an MC with unequal bias in it. Before it even gets to the air packs.

Lino, the purpose of the brake system is to split the front axle from the rear tandems to increase safety. It's hard to tell in the pic that clinto posted, but that's the case. Section 11-1 A. outlines this in the TM.

IMHO, it'd be best to replace both airpacks with like units. But if it were me, I'd use the larger volume pack on the rears since there's more volume there. Still best practice to replace with matching parts.

You said your airpack stud is broken off, I assume you're talking about the studs that are welded on? A competent welder should be able to repair that for you. Or are you talking about part of the casting ears where the bolts go in to hold the assembly?

I recall several threads describing "diagonal" braking with this system.

A search for "diagonal brake" should find what I'm referring to.

Hey Patracy,

first of all, I think your truck is amazing.

The broken part is the center boss that supports the rear of the pack. My use of stud wasn't quite correct, although there is a stud in that boss. It's the casting that's broken.

The broken one is the one nearest the MC on the left side.

doghead said:

I recall several threads describing "diagonal" braking with this system.

A search for "diagonal brake" should find what I'm referring to.

Click to expand...

Actually No. The front is controlled by it's own air-pac and the rear axles by there own air-pac. There was a discussion about the feasibility of going to a diagonal system, but the cost of running two separate lines to each axle was a bit too much.

lino said:

Well, it's not abundantly clear in the TM, but if front and rear were on different air packs then one might begin to argue that it's ok to have different air packs. If they do provide different amounts of assist, the effect would be a shift in front/rear bias. These aren't race cars, so a moderate change in bias might not be a big deal.

Most cars split circuits is a cross pattern. I'm pretty sure I read that these split front rear.

Another note is that the A2 with split brakes has an MC with unequal bias in it. Before it even gets to the air packs.

Click to expand...

There is a built in bias between the steering axel, and the rear axles, depending on when & how the truck is loaded. The difference between the two air packs (even two of the same kind), will not be noticeable.

rustystud said:

Actually No. The front is controlled by it's own air-pac and the rear axles by there own air-pac. There was a discussion about the feasibility of going to a diagonal system, but the cost of running two separate lines to each axle was a bit too much.

Click to expand...

Hmm. Guess my first post was right on the money.

Thanks for the responses, all.

Happily this is a non-issue for me now. I managed to find 2 short airpacks for about what it would cost to rebuild them (I'm hoping I don't have to rebuild...)

But after more digging, and all the above comments, it does seem like it shouldn't matter if there were one of each type of airpack on the truck.


ciao
lino