Baxi 35/60 not firing?

I have a 35/60 Baxi system boiler and I am having trouble with the boiler not firing. Initially narrowed the fault down to the pressure assembly diaphragm because the micro switch was not operating. I dismantled the assembly, cleaned and changed the diaphragm. However still no joy, the pin hardly moves out or back in. I have flushed the system twice and dismantled and cleaned out valves and pipe work on the bottom end of the boiler again to no avail. Finally swapped the pump. But still no joy. Anybody have any ideas? Frank