I purchased new callipers from a Triumph/MG 'shop', no idea who the manufacturer was, it was certainly not Girling. When fitted I had a similar problem, air in the system and no leaks. With much force you could actually press the break pedal down to the floor. I contacted the 'shop' who told me I had been supplied with the incorrect nipples, they sent me some more nipples, a little longer, I wasn't convinced and still the issue remained. I contacted them to complain questioning the build quality of the callipers (castings) and they basically told me to 'go away'. I eventually rebuilt my old Girling callipers and pleased I did. This tale doesn't really help you, however it's just to let you know I had air in the system and no leaks.