Master Cylinder - Cover Leaks

I am in need of a new master cylinder because it is contenting leaking out the front, also I have to pump up the breaks all the time to build up pressure, and I am sure it is because of the leak, the front will run dry about one a year. It has front Disk and drum rears manual brakes. Can anyone please help me decide what the replacement part to order, I also would like to know is the smaller bore ones best for Manual Disk - Drum brakes