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 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 a 2.8 inch single piston caliper. These are the same calipers I currently have.

2) Uses a Mopar rotor and a 2.375 inch single piston caliper.

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 the smaller piston would take less fluid to move and would be a firmer pedal, but I do not want to do that at the expense of stopping power.

The 2.8 piston will give more clamping PSI as the piston size multiplies the applied pressure. However, pad size, compound and a bunch of other variables come into play. You might want to figure out why the pedal feel is spongy and The brake ineffective.

Some light reading for you - i suspect a change in master cyl may fix your current issue

https://board.moparts.org/ubbthread...-math-that-you-never-wanted-to-know.html

I just went thru the same sort of problem. I built a custom front disc set up for my 58 Powerwagon. I used 12.5 inch rotors, 3 inch calipers and 1 1/8 power master. I put the power master on there 25 years ago to replace the single reservoir when i built the truck. It worked fine with the drums. However with the discs, the pedal went to almost the floor and the truck would stopped. I bled them multiple times and wasn't happy. So I was thinking the pedal ratio for manual drums are 6 to 1 and for power it is 4 to 1. I had never thought to change it. A faster ratio would reduce pedal travel. It took less than an hour to drill a new hole and adjust the plunger rod. It solved my problem and now the brakes grab better than my daily driver.

Thanks for all the replies. However, it is all still clear as mud. From what I have been able to gather from your responses and responses on other sites I have made the decision to go with the brake kit with the Chevy style rotor with Mopar 4.5 inch bolt circle and 69-77 GM calipers with a 2.8 inch single piston.

Unfortunately I have no part numbers or anything from what the previous owner used when he put together the front brake system. I was able to take casting numbers off the calipers and figured it out through a google search then compared the photos to what is on the car. However, I have no clue what rotors are on the car. Purchasing the full kit will give me a new start from hoses, bearings, rotors, calipers, pads and everything else. This way at least I know what I am working with. Once I get it all installed if things still are not performing like I think they should I may try throwing the 1.125 bore master on.

if it still doesn't stop as well as you think it should, you want to go SMALLER on the master cylinder bore, not larger.
going from a 1" to 1 1/8" bore is a huge jump in the wrong direction.
you would want to try a 15/16" bore master.