NBS master cylinder Swap

[Schurkey]

Look on the sticker in the glovebox. You've most-likely got JB5. The little JN3 brakes were discontinued a few years before your truck was made. Those had a 1" bore master, with a 34mm or 36mm QTU at the back, a smaller booster, smaller pistons in the front calipers, and thinner rotors.

If you have JB5, you had a 1 1/8 bore master with the 40mm QTU.

JB6 was the same master and booster, same front brakes as JB5, but with 11.x" Duo-Servo drums in the rear instead of the shiity 10" leading/trailing shoe drums.

 

[Zach Ondash]

How can I determine the factory bore diameter as well as the brake code(ie; jb5, jb6, etc.)

Or what if I just delete the abs entirely? I have bigger tires so the vss isn’t going to play super nicely with the abs. Would save me some money on having it bled somewhere with a tech 1

 

[MrPink]

In my case you delete ABS, Power Booster, and redo all the brake lines in one fowl swoop. NBS master, Wilwood prop valve, Ni-cop lines. But I am building a Drag/Street truck so a wild cam and turbo setup wont produce enough vacuum for my brakes. And no I wont be running a vac pump

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[jgreen16]

This may be something to do with my '90. About a week ago the pedal went real soft suddenly. Checked the fluid level in the master, and the front reservoir was basically empty. I don't see any leaks anywhere.

 

[Schurkey]

In my case you delete ABS, Power Booster, and redo all the brake lines in one fowl swoop. NBS master, Wilwood prop valve, Ni-cop lines. But I am building a Drag/Street truck so a wild cam and turbo setup wont produce enough vacuum for my brakes. And no I wont be running a vac pump

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What calipers? The pistons better be enormous.

I had a '75 Dodge Dart with manual front disc brakes. Took a lot of pedal effort to stop the car. I'm really concerned that your big-diameter master cylinder, with no booster, is going to have very high pedal effort.

This may be something to do with my '90. About a week ago the pedal went real soft suddenly. Checked the fluid level in the master, and the front reservoir was basically empty. I don't see any leaks anywhere.

Front reservoir = smaller reservoir = rear brakes. Rust hole in the brake tubes? Failed wheel cylinder(s)?

 

[jgreen16]

What calipers? The pistons better be enormous.

I had a '75 Dodge Dart with manual front disc brakes. Took a lot of pedal effort to stop the car. I'm really concerned that your big-diameter master cylinder, with no booster, is going to have very high pedal effort.

Front reservoir = smaller reservoir = rear brakes. Rust hole in the brake tubes? Failed wheel cylinder(s)?

That was what I was concentrating on when looking for signs of fluid leaking. After topping it up, and losing it again, I'm not finding anything.

 

[Schurkey]

If it was the rear reservoir, it could be leaking directly into the brake booster. But if it's the front reservoir, it's pretty-much got to be an external leak.

 

[jgreen16]

If it was the rear reservoir, it could be leaking directly into the brake booster. But if it's the front reservoir, it's pretty-much got to be an external leak.

And, for what it's worth, the pedal suddenly went almost to the floor when pulling into a parking space. Up until that point, everything was fine. Seems like whatever it is let loose then.

 

[MrPink]

What calipers? The pistons better be enormous.

I had a '75 Dodge Dart with manual front disc brakes. Took a lot of pedal effort to stop the car. I'm really concerned that your big-diameter master cylinder, with no booster, is going to have very high pedal effort.

Front reservoir = smaller reservoir = rear brakes. Rust hole in the brake tubes? Failed wheel cylinder(s)?

2.94" piston, 6.78" squared for piston area, LSMFG rear disk setup with the GM metric calipers. pedal ratio of ~6.5:1 according to Dave @ manualbrakes.com who made the bracket. I have an adjustable prop valve too so I should be perfectly fine. If I run into issues Dave stated that he would help me figure out what to do to correct the ratio and what to do moving forward.

 

[xAFNYx]

I'm so confused.
I just want my brake pedal to engage much sooner than half way depressed lol.

I bought this https://www.dormanproducts.com/p-19359-m630031.aspx because I've read everywhere that upgrading the master cylinder is the answer to this as well as adjusting the shoes outward a bunch.

I haven't yet done the install and adjustment because I read this thread and now im afraid of it not stopping haha