I'm based UK too and have EXACTLY the same issue with my 'spare' GY200 - no back brake!

Stripped the cylinder, cleaned and reassembled but still no joy in bleeding it, I've even tried removing the flexible hose from the cylinder end and cannot get fluid to pass even the cylinder itself.

I assume from this it is the hydraulic seals within so gonna try and locate a 2nd hand unit as cheaply as possible so if anyone has one spare please let me know - or in fact any other info on this seemingly simple but annoying problem.

Andy

PS

I suppose a vendor for a replacement seal kit would be a silly question?