What size O-rings are you using on the backing plate breather holes? TC88B. I've been using the #40 ( 3/4 OD x 5/8 ID x1/16) from Home Depot. aw

 

When you go to your local HD store and you look behind the parts counter and there sits a Big Sucker kit and you ask for the orings for the backing plate, they aren't going to have the ones that are originally supplied with the kit. They will bring out their o-ring choices and you'll have to just match them up but you won't find the exact size supplied with the kit. Now, you'll find one that looks perfect but actually it's a little fatter and it will pinch badly on the first install and then you can choose the one that looks perfect but actually it's a little smaller and it leaves you wondering if it's really sealing and I don't reuse the o-rings. By the way, one oring at HD was $2.36. I know it's just an oring but I'd like to see what everyone else is using. There's not a part# associated with the original o-ring. I'll call or email Arlen Ness this morning.

 

The o-ring is a pain in the butt, not metric nor standard, I had a hard time finding the right one, the o-rings that came with the big sucker were way too thin, they do leak. I end up getting a box of assorted o-ring and find the one I need.

 

Here is my reply from Arlen Ness. The o-rings are available with part# and size.

#H-024 = BANJO BOLT O'RING, #016 BUNA / $.13 each

 

As it turns out, the BUNA #016 o-ring is exactly the same size that I've been using from Home Depot. Home Depot is a #40, Danco brand but I'd almost swear that it is thinner than the BUNA brand. Looks like that I can get a pack, 100 count, for no more than $3.00. I saw them available on a few sites, plus eBay. By the way, I don't need 100 o-rings.

 

You might check out a bearing and/or hydraulic supply store. Our local Bearing House has more O rings than anyone. Bet one will work.
On edit: Farm equipment dealers have tons of O rings also.

 

buna is the material of the oring. buna-n (nitrile) oil and gasoline compatible. #016 is the standard oring part number as in AS568 @2-016 thickness is actually .070" (1/16" nominal). like previous poster said any hydraulic hose supplier should have