My short air pack is different than all the others?

[lino]

This would figure, but at least what I have is in good shape. Since I can't replace all the seals...

So, the back story:

1988 USAF deuce. Dual circuit brakes. Two long airpacks. One has broken mounting stem.
Decided to source 2 short ones (used) and rebuild. Then replace both long ones with shorts (while I'm replacing the MC also).
Got the two off ebay from a guy who has been selling brake systems pulled off A3s out in Florida. I just bought the airpacks, not the whole deal.
Got them and the externally were in good shape. Aside from a poorly converted (or originally installed) air-side brake light switch that stripped the pipe threads in the airpack housing... But I can fix that (barely).

When I opened them, they were very nice. No corrosion, lots of clean original grease. Only issue was that one of the brake fluid bores had a little rust. Water in brake fluid presumably.
Since I purchased two rebuild kits, I decided to rebuild both anyway.

I honed the small bores and that worked well. Then I honed the large bores, just for good measure, even though they were clean and original. They were also wavy. The hone pointed this out quickly.
These are pics after 15 minutes of honing. The black streaks are the "valleys" that the hone can't reach.


Anyway, based on gut feel, I think the bores will be OK.

So I started cleaning the silicone grease off the parts and identifying the new parts from the rebuild kit and where they go. And I found a mis-match.
All the short airpack pics that I've seen on this site have a one piece "upper piston" that looks like this (pulled from other threads)


The seal in the rebuild kit is sized to fit around the lower, large diameter.

Mine on the other hand are 2 pieces, and look like this:


So, where the one piece have a single seal, I have an o-ring (on the larger ring) and a wiper seal on the smaller part. I believe only the inner part moves.
No seal or o-ring for those in the kit. Also appears that this airpack activates the air-assist sooner, but at higher line pressure? Due to smaller piston diameter.

Anyway, the seals look usable, so I'll carry on.

Just wanted to share with others that there appears to be 2 variants of the small airpack.

Getting ready to get started, though I decided to strip paint off all parts since this pic.

 

[rustystud]

Did you by chance measure the piston ? It would be nice to know how much smaller then the normal one it is. It would apply at the same rate as the normal larger piston. Remember, the hydraulic piston is moving this piston. This piston is just opening the air valve allowing the boost piston to apply. It's just a interesting difference.

 

[Bunkerbuilder]

Did you measure the bore of the items you honed and do they have any taper, are you purchasing pistons that are oversize to accommodate the material you removed?

 

[rustystud]

Did you measure the bore of the items you honed and do they have any taper, are you purchasing pistons that are oversize to accommodate the material you removed?

The pistons are one size only and the seals also only come in one size. The air chamber side looks OK to me. It can take a little leaking. The hydraulic side is the important one. As long as he honed with a steady, none stop movement he should be fine. The trouble comes when people try to "get out" trouble areas by just sitting there with the hone going. Usually there is no taper in a hydraulic bore as the piston is not rapidly moving in and out like a engine piston. What usually happens is you get a rust area that eats into the metal leaving a nasty gouge that can cut the seals or allow leaking past them.

 

[lino]

Hey All,

I've attached drawings of the piston and the cylinder (the yellow anodized parts).

View attachment outer_ring.pdfView attachment inner_vavle.pdf

They are under dimensioned because I only put in the dims that I measured on the drawings. Values are also approximate. - not good enough to remake, just to give idea.
The critical missing dimension is the groove diameter of the piston seal, but the seal is in there tight and I didn't want to risk damaging it to measure the groove diameter. So it remains a mystery...


ciao
lino