To all those planning on a nbs master cylinder swap

[90halfton]

Man I'm not understanding what your asking here in sorry but user name says it all. Please explain what you are asking and hopefully we can get you some answers

What im saying is that when you let off the brake pedal the brake shoes retract, so that they are no longer making contact with the drum, meaning fluid has to return to the mc. If there is any residual pressure in the line it will expand the brake shoes. I would think that since there is such a tiny fraction of an inch between the shoes and drum (usually they slightly rub) that a tiny amount of residual pressure would cause the brakes to drag, much the same way a slightly collapsed rubber line will act.
I did this mod because of the success of others that seemed to know what they were doing, and it seems to have worked. Now I am hearing that I did it wrong and am trying to understand the need for it and the function of it, before I bolt on another thing I read about on the internet.

 

[90halfton]

He's probably referring to knuckles post on the MC swap thread. When the cylinder piston is retracted, air can seep in (past the seal I guess) unless there is pressure in the cylinder to (again, I guess) keep the seal properly seated or forced up against the cylinder wall. I would think that the residual valve must be some type of check valve that closes when downstream line pressure drops to 10-psi?


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That makes sense, allows the greater mass of fluid and pressure to return to the mc, while only retaining enough pressure to lean on a seal.

 

[90halfton]

If you pull a wheel cylinder apart and look at the cups (seals) you can see it's designed like a cup/beveled seal. The brake fluid pressure holds the outer edges to the cylinder. That's why we could get away with honing them like we did in the days before cheap replacements. Most of the time you would see a light coat of dust stuck on the cylinders because they were moist. GM said it wasn't a leak until you could push on the dust boots and get a drop or two. If you don't have the constant 10 pounds on the seal it's not really sealed to the cylinder. Maybe you get away with it and never have a problem, maybe you get air or leaks. If GM could save a nickle by leaving the valve out they would have done it


Both reasons are correct. You'll get a higher peddle if the shoes are already at the point of overcoming some spring pressure and near the drums. This is exactly why GM went to a quick take up master, high displacement to move the shoes and then a smaller displacement to help with pedal effort.

Do disc brake calipers require any residual back pressure?

 

[michael hurd]

Do disc brake calipers require any residual back pressure?

IIRC, from when I was back in high school, it was 2 psi for disc, 10 psi for drum.

 

[Ruger_556]

Is all the "benefits" to the NBS swap not just from replacing a worn out MC? Every gmt400 I've owned or driven with decent brakes has really pretty good pedal feel and can lock up all 4 wheels on dry pavement (ABS sucks but it is what it is).

 

[df2x4]

Is all the "benefits" to the NBS swap not just from replacing a worn out MC? Every gmt400 I've owned or driven with decent brakes has really pretty good pedal feel and can lock up all 4 wheels on dry pavement (ABS sucks but it is what it is).

I've been saying this for quite a while... Both of my trucks have always had great pedals except the one time that I got my Suburban's brakes bled at a shop that didn't have access to a proper scan tool. Had my preferred shop bleed them with a Tech2 and that problem disappeared. IMO if your brake system is properly maintained there's really no reason for all these modifications.

 

[Knuckle Dragger]

Do disc brake calipers require any residual back pressure?

Only if the master is below the caliper, then you use the 2 pound valve. I can't think of a single factory application for a residual valve on discs.

 

[thunderstruck]

Is all the "benefits" to the NBS swap not just from replacing a worn out MC? Every gmt400 I've owned or driven with decent brakes has really pretty good pedal feel and can lock up all 4 wheels on dry pavement (ABS sucks but it is what it is).

This, I just put a new GMT400 master cylinder (under warranty) along with new rear wheel cylinders and hose. Flushed the old fluid out and readjusted the rear drums. I seriously don't understand why people ***** about the brakes on these trucks. Pedal feels good and truck stops great.




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[beardeddummy]

It all goes back to the back brakes mine are not adjusted yet and pedal is spongy even with new 1 ton master cylinder. But were alot better last time i adjusted the rear it improved my pedal feel way more than you would think

 

[Ironhead]

My knowledge may not be all it should be, so forgive me if this post is out of line. I can appreciate that brakes are probably the most important system in our trucks. We've all had to stop quickly at one time or another. I always thought that disks were superior to drums, except that they do not do as well when it comes to emergency brakes.

If I wanted the best brakes possible on my 98, wouldn't it make sense to swap over a 99-04 rear half ton axle (disk), along with the MC, and proportioning valve? If I did this, would the e-brake cables hook right up, or do they need to be changed as well.

Any and all comments would be welcome.