The power disc brakes on my 37 Plymouth coupe have felt soft since I have owned it. I have a dropped tube axle with the 37-48 Ford round back spindles.

I bled the brakes a couple of times without much improvement. My 67-76 Corvette master cylinder had a very slight leak so I replaced it with a new unit which was bench bled before installation. Helped somewhat but not enough to say that I am comfortable with the performance of the brakes.

Looking at a couple of different disc brake kits thinking that I am going to just go new with everything including rotors, calipers, bearings, hoses etc. Finding two variations out there.

1) Uses a Chevy style rotor with Mopar 4.5 inch bolt circle and 69-77 GM calipers with a 2.8 inch single piston. These are the same calipers I currently have.

2) Uses a Mopar rotor and 78 up single piston calipers with a 2.375 inch single piston.

My question is would going from a 2.8 inch piston to a 2.375 inch piston make much of a difference? I think it should make the pedal a bit firmer but will it effect the stopping power? The master cylinder is a 1 inch bore by the way.

One reason I am thinking of using the kit with the smaller piston is due to the stock Mopar rotor being more readily available compared to the Chevy rotor with the Mopar bolt circle. The other would be what I believe would be a firmer pedal due to the smaller piston, but I do not want to do that at the expense of stopping power.