I recently replaced the master cylinder on my 71 cutlass conv. The car has power drum brakes all around. The master cylinder that I took off had two equal size reserviors and was reletively small. The master cylinder I replaced it with was a little bigger and the front reservior was bigger than the rear.
My brakes work but they don't perform like I would like them to. I don't know if it just the nature of drum brakes because I have never owned a car with drum brakes front and rear or maybe the new master cylinder is not right?
I told the autopart person that the new and old did not match visibly but he said as long as the bore on the rear of the master was the same then it will work. Is that true? I think the new master is for a car with front disc and rear drum judging by the different size reserviors. Is this safe?

 

9 times out of 10 you can rebuild your original with a 15 dollar kit.
I'm not sure about the rear bore thing. If I was not keeping the car original, I'd go for a real nice aftermarket one. If it doesn't end up working out for you, I have a Master cylinder from a 72 delta kicking around here somewhere.

 

Bore does make a difference. I don't remember the laws of physics describing fluids under pressure but do remember that innner diameter of a device that handles fluids affects the fluid pressure. So, if you have a m/c with a different bore than the original one the pressure will be different than intended originally, which means in your application the brakes will "grab" too quickly or slowly. You said they don't perform like you would like them to, this may be the reason. You might want to research this a little and perhaps get a master cylinder that is a direct replacement of correct bore.

 

I have a 72 Cutlass convertible with drum brakes. It is the second one I have owned and they both have had 4 wheel drum brakes. I replaced the master cylinder and then had a wheel cylinder start leaking. I gave up and am going ahead with a disc brake conversion. I had the master cylinder with equal reservoirs and got one like you described. My opinion for what it is worth is that if you go and get the master cylinder they list for manual drum brakes, it will have the same size reservoirs front and back. I think the one with the different sizes is for disc brakes. I just kept it since I am doing the disk brake conversion. Be sure and bench bleed the master cylinder or you will never get all the air out. I learned that on my first Cutlass about 25 years ago.

Good luck