Back wheel seized up?

After leaving my bike for a few weeks, the back wheel had seized up pretty badly. When i first bought it, it was in a similar condition but i handed it off to a mechanic so i don't know what happened. I presumed it was just the brakes. I took off the caliper and pumped out the pistons, cleaned and lubed them. They now work flawlessly. Everything was fine until I fitted the back wheel, then the pressure returned and the wheel was hard to turn. When the brakes were assembled, the hub spun freely, it only seems to be difficult when the wheel is on. There's no play anywhere and it doesn't seem to be a bearing.

Any advice would be very helpful.

 

I take issue with your last staement that " any advice would be helpful".
I think that what you really mean is any helpful advice would be apprieciated.

But I digress.....
If you remove the rear tire and rim, and the hub/ brake disc assembly spins freely, then it sounds like the tire or rim are against something when installed.

What year is the bike? We are assuming 1990 - 1997 because this thread is in the 3rd and 4th gen Forum.
How many miles/ km on the clock?

 

spacer m issing ?

as a test, loosen the rear axle nut about 1 turn then try wheel again. remove brake pads unless youre sure of no drag.

 

"Leaving for a few weeks"- ? Outside? What did the mechanic say about it, assuming he worked on it?

 

There's no drag no the hub, the hub turns freely until I put the wheel back on. The only thing I can think of is that the bearings are gone and the wheel is sagging enough to play against the disk.
She's inside but she's a project so she's not being run. She currently doesn't have a seat. I can't speak with the first mechanic. I'm in Cambodia and picked this bike up from a province a long way from the city. The first guy tried to steal the bike so I never stayed in touch after getting her back. Things are different here.

 

Sounds to me like the weight of the wheel and tire is causing some deflection in the axle. To me that spells bearings. Take the wheel off and check for any play in the hub.