Richard - is the new brake master cylinder coming off OK?
If that's what you've changed last, start there first. There's a valve somewhere in the system to allow the piston to return fully. You may need to search the internet for details.
Where there is a servo, there is usually an air filter (for the diaphragm) and if it gets clogged the brakes can stick on. So if you have a servo, check that.
