Caliper Piston Size

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.

The bore size ( diameter ) has no effect on whether a brake pedal feels soft or not.
A soft or spongy pedal comes from brake hoses "swelling" under the pressure and/ or air in the system. Pumping the brakes to bleed the air out will actually cause the air to stay entrained in the fluid. So try to limit pumping the pedal as much as possible. Also, some years back there were bulkhead fittings that because of their shape would hold air at the top of the fitting. These were used to pass through the frame typically, there was a "raised" section in the middle that would actually act as a reservoir and hold air.

The larger diameter brake piston will give you more surface area - so more "brake horsepower". Always a good thing!

pedal ratio is very important . one thing i have found over the years is trying to push air out with the m/c is a toss of the coin. at times i have raised the rear of the car as high as i could to get any trapped air to travel to the rear.
as stated dont pump the pedal. this will break up any air bubble and aerate the fluid . a power bleeder is your best bet .

Another possibility, brake fluid absorbs moisture from the air (the reason for that funky, flimsy gasket) and over time causes fluid in the lines to get thick and nasty. Flushing the lines as you bleed is a good idea, and may solve your problem if you see thick, dark amber colored fluid being expelled.

small bore m/c gives you more power , larger bore gives you shorter pedal travel .

Depends on the type of brake fluid about moisture absorbsion--------and the amount of moisture also then changes the boiling point temp of the brake fluid which can be echhhhhhhhhhh in high performance type aplications or industrial , military type stuff-------

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Originally Posted by jerry clayton

Depends on the type of brake fluid about moisture absorbsion--------and the amount of moisture also then changes the boiling point temp of the brake fluid which can be echhhhhhhhhhh in high performance type aplications or industrial , military type stuff-------

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Talking plain old DOT 3 fluid, Jerry, and not in any high performance application, but DOT 3, 4 and (Correction DOT 5.1) are all hygroscopic. Just good practice to flush the fluid once in a while.

yeah i misread it also . i was talking m/c . but same effect. smaller bore caliper will take less fluid to travel which will effect pedal travel some. if the m/c is in the floor most all of them have piss poor pedal ratio . if it is on the firewall the m/c rod can be dropped 1 inch and it will give more pedal but slightly less power . i have had good luck with s10 booster/master cylinders with standard gm calipers .

I didn't misread it, but was just adding to the discussion that Mike (34_40) and shine had offered about what might be causing the spongy pedal regardless which caliper is used, and responding to Jerry about fluids and moisture. I think shine has answered the difference in caliper piston size, so it's all good, but thanks for clarifying the question, Denny.

Dot 5 is silicon based and NOT Hydroscopic. Dot 3 & 4 are glycol based and are hydroscopic

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Originally Posted by jerry clayton

Dot 5 is silicon based and NOT Hydroscopic. Dot 3 & 4 are glycol based and are hydroscopic

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Sorry Jerry, that was supposed to be DOT 5.1 but I missed the suffix! You are absolutely right, DOT 5 is the purple silicone based fluid. It's a moot point anyway as we got off base from the question asked.

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Originally Posted by hkestes

The power disc brakes on my 37 Plymouth coupe have felt soft since I have owned it..

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.

I think it should make the pedal a bit firmer but will it effect the stopping power?

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.

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You really think he was asking about a ratio?

not a lot . the 2.8 will require more fluid / pedal travel . brakes drive me nuts and i was a hyd tech in the navy .
larger the bore in an actuator the more fluid and psi to move it but it will have more lift power .
on power brakes the larger dia of booster the more lbs of boost, or a dual diaphragm . those little 7 in boosters are useless . a smaller m/c bore = more power but more travel , larger bore m/c = less power but also less travel .. pedal ratio is more important . if your pin hole on the pedal is 2 in from center you can redrill at 3 in and get less travel and more power .

hope this helps .

Thanks for all the replies. However, it is all still clear as mud. From what I have been able to gather from your responses 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 you run the gm calipers an s10 booster m/c will work fine .
this truck had a gm clip and complete s10 pedal / booster / m/c . great brakes .

I like your reasoning. And items such as the hoses can have an impact on the "firmness" of the pedal feeling. Plus like you said, you'll know exactly what you have in place, no guessing now.

i like to run stainless hoses . not too much more . but if the system is right it should not matter . every time i get into that after market crap i have problems. soft pedal , no power etc . worst is under floor systems .

And I have an under floor booster/master.

your stuck with the small booster and bad pedal ratio . some folks say they have great brakes but it has eluded me on many cars with under floor setups . wish i could help .

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Originally Posted by DennyW

Have you checked The booster pin to master cylinder gap ? :)



https://techtalk.mpbrakes.com/how-to...r-cylinder-gap

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No I haven't but I will now. Just ordered a tool to check it with.