[Healeys] Brake servo

[Gaagten]

Hello,
Yesterday I intended to go for a ride. Taking the car out of the garage I
noticed a strange behavour of my brakes.
The pedal felt very hard and not too much breaking was there. I just drove
some blocks and nothing was chanced. Pushing the brake pedal several times
resulted in blocking of the front brakes and the car could not be moved
anymore).
However after a little time the brakes released themselves again.
My conclusion is that something is wrong with the brake servo. The servo is on
the car as long as I have it (almost 15 years, about 20.000 km)
So, if I have to replace the unit, which servo type should I take. I have
probably a girling now. I know that the substitute for this is a Lockeed
servo.

Any other idea

Thanks and regards,
Ge Aagten
BJ8
The Netherlands