Trapdoor Rifles

I have not found the issue yet , I am sure , it's gonna be a mismatch in parts , or maybe a out of spec part in the lock , only way is to start replacing something, I will probably replace the sear first and see how that does , it has a 3 position tumbler, so it could be the sear is incorrect, just ...

It sets, but drops when trigger pulled , i will post the pictures of lock later today as I am not home at the moment

I have a model 1873 trapdoor that has been sporterized and has a mixture of parts ,which is typical, my problem is this , at half cock ,when trigger is pulled, it will drop , it has the 3 position tumbler, is this a typical problem that shows a part is worn past tolerance, if so , what so I look at ...