Unexpected Rear brake cylinder size

We have started upgrading the brake lines and replacing the single master cylinder.
when I took the back brakes apart I noticed that the cylinders were definitely not the7/8"
or 1 1/16" that we had expected. They have offset bolt pattern like the fronts and when I took them apart they are 1 1/8" bore. I  have searched a Bunch and cant find any reference to that size for these vans and the part number on the cylinder doesn't show up.
also the 1 1/16" Cylinders and rebuild kits seem to be scarce.
Any suggestions on what size cylinder would fit the backing pates I have on the rear?
Rear end is a 12 Bolt, brake shoes are 2 1/2" wide.
could just be PO with some Frankenstein parts?


pics posted

The 1st gen Chevys have the metal line go straight in and 2 bolts above them,,,,,,so lets see if someone has the rear wheel cylinders with the offset inlet and diagonal bolt holes,,,,,,sure looks like a front wheel cylinder for a rubber hose but those parking brake cable holes make it a rear backing plate,,,,,,,,,from,,,,,something,.

Thanks donivan
I think I will try to rebuild the cylinders I have here First but may need to
get a hold of some proper backing plates eventually to get everything back to
normal.
It will be interesting to see if anyone else has seen anything like this before.

There are variables,,,,,The 64 Chevy vans had the 10 and 12 bolt rear ends,,,,and 5 slot grilles,,,,then they called the leftover ones 1965's until about May 1965 where they  then  made  6 slot grille ones, changed the wheel cylinders, called them 1965's also  and a added  a SportVan model,,,,,and what was GMC doing at this time,,,,,they added a HandiBus model,,,,,,the GMC were rated as a more heavier duty vehicle than Chevys.
So I guess what we need is some information from GMC van owners,  does anyone see diagonal bolt holes and an offset brake line when they look at their rear backing plates on their vans like the one on the right here?  

Pics of 52 year old brake cylinders off my 68 G-20 van. (wondered Why they didn't work too good??)
Had to down size to next closes size($25. EA). (They wanted $255. EA to sleeve them)

So more variables,,,,,GMC has Step vans and pick ups and there are some settings that only apply to vehicles made in CANADA,,,,,,,,,,but here is a parts listing for GMC.....

Certainly looks like there were a lot of variables in the 64-65
Range. Donivan I think you are onto something with the fact that
This Handivan was manufactured in Canada, could be the other 
missing variable here.

So whats the plan,,,,,,see what is on the front,,,,,,,see if it is 1 1/8",,,,,,see if it has the same bolt pattern,,see if hose connection can be adapted to metal flare,,,,,,,order rebuild kit for the front if its 1 1/8" and rebuild rear cylinders,,,,,,,,,if,,,,,,,,,,rust was sleeping....

Ah - I have a brother on the rear axle - Nope, not so much a Frankenstein by PO
Let me ask/guess is your van is 1965 with 4:11 (2900 lb) rear?
On the upper side of your cylinders there maybe a date code cast in - that may give a hint if the Ø1-1/8" cylinders are later installed parts.

I have a 1965 parts book & Ø 1-1/16 cylinders are listed for the van.  Note: The 3:36 rear axle vans are different

I pulled my parts off & rebuilt them. I have 1-1/16 all around (Delco parts) diagonal bolt patterns.   With the backing plates having the diagonal bolt pattern not so much a though of a mod by a PO.

For cylinder P/N's the tricky part is that the casting numbers are not the same as the GM cylinder assy numbers.

For example wheel cylinders assy for the van
  Left: Assy 5465445   <--  {casting # 5462965}
Right: Assy 5465446   <--  {casting # 5462966}

For Ø 1-1/16" rebuild kits:
1-1/16"  GM 5465084 | 173-116 | 173-127 | 906-VP
1-1/16"  ACDELCO ACD 18G29 | 18031208 - Not marked Delco; has 49262 (same as ORielly's BrakeBest)
1-1/16"  BrakeBest BHH WC46349
1-1/16" Dorman R823G

Don't think you need it - I have a listing of Ø1-1/8" P/N's with the same "diagonal" wheel cylinder bolt pattern - PM me your email address & I can send you documents.

On the subject for the rear shoes, the 2-1/2" wide ones are correct - P/N 3846099.   I went to great lengths to run down a new set (the auto parts places offer 2" for the rear.) All said and done I did a rare parts search & found some NOS P/Ns.  After all that I matched up the shoes & I will be damned they were IDENTICAL to the front 2-1/2" GM shoes P/N 3886395.

Just for discussion it struck me as odd that the rear shoes were the same size as the fronts.  I expected smaller braking component in the rear.   I can only speculate that GM balanced both the front & rear cylinders the same size & balanced the same size brake shoes.  Maybe some consideration for the low rear axle gear ratios - I don't know...

Let me know if you need more data...

Thanks for all the info gfleduc 
It’s good to know that there are more like this out there!
And this morning I checked the rear ratio at guess what …4:11
I dug through a bunch of receipts the PO gave me with the van
And it looks like all cylinders were swapped out at one point or 
Another so no original parts. I did pull the fronts as well and 
They were 1 1/16” bore. As for my plan of attack I have 
All new for the front and have ordered a rebuild for the back.
If they don’t work out I would consider putting the front hose
Style on the rear with a hose fitting with crush washer to flare.
I will pm you if I need those other documents you mentioned.
Mystery may be solved!

Well - you may be lucky to know I have been watching these for over a year - I was watching in case my rebuilds did not hold up.

https://www.ebay.com/itm/162725979000?hash=item25e3396378:g:iNQAAOSw-5dZ8Wa6

Was not sure - Question do you mind sharing photo's or the numbers on your fronts (if they are the originals...  Also your master cylinder is you have it off the van.

Nice work on the EBay find.
Hopefully neither of us will need them but who knows.
You get first Dibs.
I have attached master cylinder part number it is in pretty rough
Shape but I think it is G4 57
Also the fronts aren’t original but I took a pic anyhow.

Thank-you for the photo's