piston in caliper

Hello, been lurking for awhile and am new norton owner with a 75 ES with 8k on it and very original but sadly neglected. I'll be starting full strip down in a few days with a friend who has done several but having a problem getting the pistons back into the calipers past the seals. I'm using all new parts and stainless pistons. I'm lubing everything with brake fluid, calipers are clean and managed to get inner piston past the seal with lots of force but afraid the seal may be cocked or something.Also can't move it without air pressure to blow it back out.
should they be hard to move? I know they will move with brake fluid behind them but worried will they retract or just drag on the disk. All my experience is with old BMW's so looking forward to this rebuild. Just wondering if there is a secret to this. This bike has been sitting for 30+ years and I'm intend to do most of this myself except eng and tranny work which my friend will do with me watching and learning( and paying!) Thanks Arch.

I'm assuming when you say "all new parts", that you're probably not talking about the caliper itself. If you are reusing the caliper, make absolutely sure the seal grooves are completely clean and free of any accumulated deposits. It doesn't take much in there to pressure the seals against the pistons enough to make them a very tight fit. Over the years I've made scraping tools and, with a mirror to check progress, very carefully clean the bottom and sides of the seal grooves in all types of calipers. Can make a big difference.

If you haven't already done this, I hope it helps.

I had the same problem your having now and to tell you my fix may be taboo, but I removed the seal and with caution I stretched it Very little all around and then placed it back in. The piston fit and the seal retracts back to size. Iv not had a problem to this day with leak or braking. This is only after I tried to much force one time and chopped a seal, so ya dont force it. (insert bad joke now)

But like I said this is just what I did, theres a lot of smart guys on the site id see what other advice you get.

Pack some 60lb fishing line in the cavity before you insert the piston, apply a bit of pressure to the piston against the seal, , then slowly start to remove the fishing line covering the full circumference of the piston, try not to pull to much in one position as it may put a groove in the seal face. It basically lifts the leading edge of the seal up and over the leading edge of the piston.It works though as I have done this before.

Have just been down to the workshop and doing a piston and seal replacement for Bike #2's caliper.
I used the end cap to push the outer piston through the outer seal, just by applying slow even pressure when screwing the end cap in, in turn applying pressure to the piston. You can use the same method wiith the inner piston, sit the piston sqaure on to the inner seal, having applied a light smear of brake fluid or silicone grease(i mean shiny only ) then cut a lencth of tube(pipe) that sits on the dished end of the piston and is long enough to just sit inside the caliper, then screw in the end cap and slowly applying pressure on your piece of pipe, in turn applying pressure to the inner piston and press through the inner seal. You can't go to far with the inner, as soon as you fit it and apply pressure from the master cyclinder, it will pop out to its final working position up against the disc.
Hope this makes sense!!!
Regards Mike

holy crap!!!

To much information do you think???
Here is a mod I did today on caliper as PO('s) had damaged the end cap holding on the pistons.
Drilled and tapped to 6 mm then countersunk holes to clean up PO(s) mess.
Fill holes with c/s screws. Might paint it black , what do you think ?
Enough info?
Regards Mike

Three things, never use force to replace pistons in callipers, and clean out the grove(s) for the seals first, and always renew old seals, use a grease like Loctite high melting point silicone grease.
In my experience, avoid copper grease, because I have found that it is too gritty and will mess up your dust seal outer surface within a week of daily usage, don’t ask me how I know :!:

Thanks for all the replies, I'll start over today and even though I thought I did a good job on cleaning the grooves I'll double check them. I ordered another seal kit from a different supplier just in case I need it. Silly me, I thought the hard job would be to get the calipers apart after 30+ years but that was easier than getting them together!! thanks Arch. Oh and I measured the new ss pistons against the old ones and new was about half a thou larger than old so that would have to make a little difference I guess.

Sucess! I stretched the seals ever so slightly until they were almost flush in the groove and the inner pushed in with hard finger pressure and I used the tip about using the end cap as a press to push in the outer piston and worked like a charm. Now I can move the pistons with solid finger pressure so I feel much better about things. It took about 5 minutes total for the front caliper so thanks for the tips and help. Arch.