How to: NBS master cylinder swap for firm brake pedal

[df2x4]

If some idiot installed a non-quick-takeup master cylinder on an application with low-drag calipers, the pedal will be low.

Question, what would be the easiest way to diagnose that this mistake was made? I have a suspicion that an incompetent dealer tech may have done this on my Suburban years ago but I have no idea what to look for. The brakes work great, but I have to move the pedal much more than I do in my red truck. It never used to have that much travel.

 

[Schurkey]

Question, what would be the easiest way to diagnose that this mistake was made? I have a suspicion that an incompetent dealer tech may have done this on my Suburban years ago but I have no idea what to look for. The brakes work great, but I have to move the pedal much more than I do in my red truck. It never used to have that much travel.

Does the replacement master cylinder have a bigass step at the rear of the casting like the one in the photo in my post?

There's also the issue that you may have a 1" bore, 36mm QTU cylinder in an application that needs a 1 1/8 bore, 40 mm QTU master cylinder. I don't know of any way to check that except to remove the thing from the booster and measure the rear opening.

Are the rear brakes properly adjusted? Do you have the crappy Twin Leading Shoe rear brakes, which require the park brake to be used regularly? Even the good Duo-Servo brakes can get out-of-adjustment if the adjuster seizes.

 

[df2x4]

Does the replacement master cylinder have a bigass step at the rear of the casting like the one in the photo in my post?

Not sure, the Suburban is with my mechanic right now getting the AC system overhauled but I will take a look (and maybe some pictures) when I go get it tomorrow.

Are the rear brakes properly adjusted?

To the best of my knowledge, yes. I let my mechanic handle that when he last replaced the shoes though so I can't verify personally. They seem to work fine once you hit the point in the pedal travel where they start to grab.

Do you have the crappy Twin Leading Shoe rear brakes, which require the park brake to be used regularly?

Once again, not sure. Is that determined by which RPO code you have for brakes? I have JB6 on the Suburban.

Thanks for the response, I'll try and get some better information once I have the truck back at home.

 

[Turbokat]

Does anyone know how the pedal feel is improved if you already have hydroboost? Is it worth making the modification to the hydroboost just to fit a NBS MC?

 

[DerekTheGreat]

...To the best of my knowledge, yes. I let my mechanic handle that when he last replaced the shoes though so I can't verify personally. They seem to work fine once you hit the point in the pedal travel where they start to grab...

Pop those drums off & get a peek under that skirt for ye self! My brake pedal was getting mushy soft to the point where it seemed the thing traveled halfway to the floor before anything happened and it got firmer. I knew my brakes were out of adjustment but whoa. Drums slid right off by hand. GM's 10" drum set up sucks as you can't adjust them manually without removing the drums or creating another hole in the backing plate. If your drums have been recently turned then you should be able to get a nice firm pedal like I did by adjusting them until you get a slight to moderate drag of the shoes on the drum when turning by hand. It'll seem like they're over-adjusted but they're not- my truck still rolls around with ease. It's a stick and since I drive old garbage daily, I know what sticky brakes feel like haha. But my pedal is firm now, much much better than it was. I like to say that the biggest and cheapest fix for "Crappy truck brakes" is to make sure the drums are adjusted. The slack adjuster provided from the factory sucks, I'm not sure they even work on their own.

 

[Schurkey]

I like to say that the biggest and cheapest fix for "Crappy truck brakes" is to make sure the drums are adjusted. The slack adjuster provided from the factory sucks, I'm not sure they even work on their own.

The Twin Leading Shoe adjustment works in conjunction with the park brake.

If you never use the park brake, you're never adjusting the brakes "automatically".

 

[DerekTheGreat]

I did not know that. The parking brake is the one thing I need to fix on my truck. It's just that even when I got the shoes adjusted, the lever really wasn't on the starwheel.

 

[Schurkey]

With Twin Leading Shoe brakes, it's like firing an old gun. The lever has to be "cocked" before the trigger can "release" it.

Stamp on the park brake pedal, the motion inside the drum ***** the adjuster lever, if the shoe wear is great enough to need adjusting.

Step on the service brake pedal, the lever is released which turns the star-wheel and adjusts the shoes.

You don't use the park brake, the lever never *****, and therefore can't be released. The star wheel doesn't turn, the shoes get farther and farther out of adjustment, and the service brake pedal gets too low.

This is entirely different from Duo-Servo rear brakes, where the lever gets "cocked" when needed, by braking firmly in Reverse. The lever releases, turning the star wheel when the reverse-torque is removed--you let off the brake pedal. Folks tend to back up a lot more than they use the park brake.

 

[DerekTheGreat]

::Insert shooting star thingy here: The more you know::

Guess I'll get on replacing the parking brake sooner rather than later. I do like to use them when they work, I've never known one which didn't freeze up when least convenient to do so.

 

[Schurkey]

When it was me, I converted to a 6-lug 3/4 ton axle that had Duo-Servo rear brakes. But then, my axle had 300K miles on it, and was pretty wiped-out anyway.

It's been thirty-five years, but the way I remember it, this Twin Leading Shoe design came from the Vega. It was **** then, and it's **** now. It got GM sued when they used it on the Citation "X" body.

The more you use the park brake, the less-likely it is that the cables will seize.