Have a slightly weird confusing brake issue. I brought a pair of rebuilt front calipers (mx5) from ebay. In retrospect probably shouldn't have gone with cheapest available. Anyway the left one leaked around the piston. Still works definitely leaking. Wanting to save the hassle of sending caliper back I asked the seller to send new seals and I wouldn't take it further. Never pulled a caliper apart before but I popped the piston out. It looks new. No scoring on the cylinder wall. Put the new seal in and...it still leaks. So what do you reckon - all I can think is they overbored the caliper?

I'm pretty sure they won't have rebored caliper, as this would change the brake force surely?and would require a over size piston.

Batch of dodgy seals would be my guess.

How easy was the piston to push back in with the new seal fitted?

Sorry to be negative chap, but I would have gone straight for the replacement. You might have scuppered yourself by agreeing to settle for the seals. It certainly sounds like the caliper is duff!

Probably score in the cylinder or a damaged seal seat , or even possibly a crap mismatched piston itself
Could be anything without seeing it but strip it and inspect carefully for any damage

If the piston is in good condition and the groove in caliper is clean the most likely causes are with mismatched undersize piston or the rubber seal is fitted the wrong way round. The wedge shaped profile of O ring that forms the seal only seals in one direction and won't work if fitted the wrong way round.

Also you don't say what type of caliper but Girling Lucas "Colette" floating calipers have a fairly generous clearance between the piston and bore and can leak if piston is tilted in the bore when pushing it back.

Guess I did shoot myself in the foot there...at least it was only £25. Having popped it out again and everything still looks good I reckon mismatched piston. Did seem a bit easy when I put it in.
Well I'll get another whole caliper from a motor factors and be done with it I think

Just measure the bore size and buy a new piston for it would be cheaper