Caliper Piston Size

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Well-known member
May 12, 2008
Plano, TX
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.
Just thinking out loud...

Sounds to me like you have a friction problem, where the pads and rotors no longer have the necessary bite. Before you throw money at it, resurface your rotors and replace the pads with OEM organic. If you've already tried that, please ignore this message. If you haven't, I suspect glaze is the root of your problem...

There's actually a way to calculate it. I can't remember it exactly but I'm sure you can find it online or you can call Wilwood's tech line. It's a ratio of the caliper size vs master plunger size.

If you use the smaller caliper you will sacrifice braking power hence, the reason all the big boys use large calipers and rotors.
A 1 1/8" master cylinder is generally used on power brakes however, a 1" master cylinder is plenty big for disc to drum and disc to disc brake systems and will give better clamping force on the system than a 1 1/8".
Some will disagree with me which is alright but, it comes down to simple hydraulics. The smaller the master cylinder the more pressure created with less effort because, of less fluid being moved each time the plunger is moved.
Manual brakes have a firm pedal but, power brakes have a spongy feel because of the booster. If you have a stiff pedal with power brakes then there is probably something wrong with the system. If you want an example just get in any newer vehicle, push the brakes with the engine running and you'll see what I'm talking about.
Excessive sponginess is a different story. There can be several things contributing to this.

Manual brake pedal arms generally have a ratio of 7 to one and power brake pedal arms have a ratio of 6 to one. Using a 7 to 1 ratio will increase the sponginess, on power brakes.
Then there's the flex lines, rubber lines expand each time the pedal is pushed where nylon with stainless reinforced coverings expand very little and gives a firmer pedal.
OK, I'm done :)
A couple other other things you can check and probably already have is drum brake adjustment in the rear. out of adjustment will give you more pedal travel and will some times pump up. the other thing I've seen is front wheel bearing set up loose which will let the rotors push the pads out when brakes are released.
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.

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