It's not the check valve, it's the servo. The check valve rarely fails and if gives another mistake. It really is the servo at fault. Removing the check valve makes your car operative but there will be no assistance to braking, so be careful if you bring your car home this way.
Thanks, don't worry we are not driving the car with the check valve removed. Once I removed it and released all the pressure I put it straight back in and the car made it home with normal brakes again.
I was going for the check valve first because it's a lot easier to try. I've sprayed WD40 into the opening just in case it's sticking and I'll try it tomorrow. I have seen mixed information on the internet about these check valves, one place said they are responsible for releasing the built-up pressure after the brake pedal is released and to me that sounds like a sticking valve could allow too much pressure, therefore too much servo assistance. But you think this operation is inside the servo, looking at your last two responses?