I feel your pain Dan, I spent several months before I left attempting to fix a spongy break pedal on Watson. Things I learned during my adventure.
1) just because a third party brake hose fits an OEM wheel cylinder, and a third party wheel cylinder fits the OEM brake hose, the transitive property does not apply and the two third party parts will not fit.
2) British engineering is designed to sabotage your efforts (i.e. if you do not know to loosen the brass brake line on the other side of the subframe before undoing the brake hose the line with twist and snap.
3) the master cylinder goes against gravity (the fluid goes out the top) making it near impossible to gravity bleed. Also one of each of the twin wheel cylinders has the bleed screw at the bottom meaning air gets trapped at the top.
4) drums brakes require manual adjusting with the difference between wheels locked and pedal on the floor being a 1/16 of a turn on any of the 6 adjusting screw.
Watson won the battle, before I left I disconnected the line running to the front brakes and installed a breeder screw at the "T". Bled the rear brakes so he had some brakes and rolled him to the back corner of the garage.
I am search the junkyards here to pick a disc system, or if I have no luck just ordering a replacement front disc kit. I want to upgrade of the front drums anyway. I will also likely replace the single line master cylinder with a dual line system. This started when I had a brake line fail, that was not fun on a single line system.
I have accepted that most projects on the classic follow the 5 stages of grief:
Denial - This will be easy. I will find the issue, replace, and bleed the system. I will be done in an hour.
Anger - This is suppose to fit, what moron designed this. Now I have to wait a week for the right part.
Bargaining - Come please let this blind bolt thread, just give me a solid pedal one time.
Depression - I am never going to get this right, I should just cut my losses.
Acceptance - I just need to replace the whole system.