[OldMan57]

I've narrowed down the two choices for master cylinders that will work for an off road toy. I have Ford dual piston front calipers and huge GM 1 ton 3.xx" single piston rear calipers. In the past I've been able to lock up the rear 37's on the street at 1/2 brake pedal.

P2424 is a Chevy master cylinder for various 3/4 ton Chevy trucks. It has a 1.125" front brake MC bore, and a 40mm rear brake MC bore. According to my calculations I'll achieve enough line pressure with the 1.125"bore for the front brakes, but a 40mm bore nearly halves the pressure to the rears. How will this feel to the driver as far as a pedal that's not too stiff but doesn't go to the floor?

M2423 is the other MC I have, which is also from a 3/4 ton Chevy and is a straight 1.125" bore MC, according to Napas website. I'm assuming this will simply apply the same pressure to both front and rear.

Which would you choose? The near future of this truck is to swap the 37's for 40-42's with no additional changes to brakes.

 

[williamj]

I would choose the m2423 and add a rear proportioning valve for adjustment.

I am not sure it would be easy or even possible to increase pressure if you needed it more in the rear.

 

[tantjjim]

Neither. I have the same setup and have been very happy with a 1.25" master cylinder. IIRC, it came out of a 2000 dodge 1500 pickup, may or may not have the same brake line thread sizes you want but I'm sure there's another applications that has the 1.25" bore if you do some digging. Pedal travel and feel with this setup was almost identical to what I had stock before the 1 ton swap.

FWIW, I also tried a 1.125" master cylinder and it worked but the pedal travel was pretty long and the pedal was pretty soft. It got the job done but was always a weird feel for me so I went with the 1.25" MC.

 

[OldMan57]

Neither. I have the same setup and have been very happy with a 1.25" master cylinder. IIRC, it came out of a 2000 dodge 1500 pickup, may or may not have the same brake line thread sizes you want but I'm sure there's another applications that has the 1.25" bore if you do some digging. Pedal travel and feel with this setup was almost identical to what I had stock before the 1 ton swap.

FWIW, I also tried a 1.125" master cylinder and it worked but the pedal travel was pretty long and the pedal was pretty soft. It got the job done but was always a weird feel for me so I went with the 1.25" MC.

What I didn't like about the 1.25" MC bore size was the drastic drop in brake line pressure, according to BillaVista's brake spreadsheet calculator. I know it can't tell me EXACTLY how much pressure a setup will need or make, but it does a fantastic job comparing apples to apples when just looking at how changing one variable effects the system. When moving from a 1.125" bore to a 1.25" bore, there's a 19% drop in calculated line pressure. That's why I thought perhaps the 1.125/40mm bore may work to give me the front brake pressure I need and the firm pedal most desire. Not sure though. I brought both home and have the M2423 bolted up right now. Should be able to test by this weekend after making new brake lines.

 

[tantjjim]

What I didn't like about the 1.25" MC bore size was the drastic drop in brake line pressure, according to BillaVista's brake spreadsheet calculator. I know it can't tell me EXACTLY how much pressure a setup will need or make, but it does a fantastic job comparing apples to apples when just looking at how changing one variable effects the system. When moving from a 1.125" bore to a 1.25" bore, there's a 19% drop in calculated line pressure. That's why I thought perhaps the 1.125/40mm bore may work to give me the front brake pressure I need and the firm pedal most desire. Not sure though. I brought both home and have the M2423 bolted up right now. Should be able to test by this weekend after making new brake lines.

I think a comparison of both is a great idea. I've known people who really liked the 1.125" MC and others (myself included) that really like the 1.25" I think both work fine and at the end of the day it boils down to personal preference. Update when you decide which one you prefer, I'm curious to see which one you like better.

 

[OldMan57]

I think a comparison of both is a great idea. I've known people who really liked the 1.125" MC and others (myself included) that really like the 1.25" I think both work fine and at the end of the day it boils down to personal preference. Update when you decide which one you prefer, I'm curious to see which one you like better.

Perhaps. If I like the 1.125" bore I'm probably not going to crack the system open again, waste a bunch of brake fluid and re-bleed just for the PBB. :flipoff2:

If I think the 1.125" goes too far down to the floor but has enough stopping power for my tastes, I'll try the 1.125"/40mm and report back. Fair enough?

 

[tantjjim]

Perhaps. If I like the 1.125" bore I'm probably not going to crack the system open again, waste a bunch of brake fluid and re-bleed just for the PBB. :flipoff2:

If I think the 1.125" goes too far down to the floor but has enough stopping power for my tastes, I'll try the 1.125"/40mm and report back. Fair enough?

Unacceptable, you must test both, so sayeth the PBB :flipoff2: