To all those planning on a nbs master cylinder swap

[Knuckle Dragger]

If you look at the swap thread you'll see I have already posted the info for the residual valve.

 

[90halfton]

The pedal feels a little better in my foot. But the big difference is in how much better the truck stops. It stops like a new car now.

 

[90halfton]

I guess I'm failing to understand how air can be pulled in without there being a leak? Mines worked great for a while, so I'm probably not going to jack with it. Wish this would have been mentioned at the beginning of the thread on how to do this.

 

[beardeddummy]

That and as i said i was wrong they should still work just not to their full potential.

 

[east302]

90halfton,

I'm not quite sure on it, either, but can reason that there would not be a leak if there was a constant upstream pressure pushing against some seal or otherwise seating some component. I see some manufacturers say that they're used to keep the shoes closer to the drums (to reduce pedal travel) and others that the slight pressure prevents fluid from coming in when the cylinders retract.

I don't know. I figured I'll use a disc/drum MC just in case.




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[beardeddummy]

I guess I'm failing to understand how air can be pulled in without there being a leak? Mines worked great for a while, so I'm probably not going to jack with it. Wish this would have been mentioned at the beginning of the thread on how to do this.

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

 

[east302]

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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[Knuckle Dragger]

I guess I'm failing to understand how air can be pulled in without there being a leak? Mines worked great for a while, so I'm probably not going to jack with it. Wish this would have been mentioned at the beginning of the thread on how to do this.

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.

90halfton,

I'm not quite sure on it, either, but can reason that there would not be a leak if there was a constant upstream pressure pushing against some seal or otherwise seating some component. I see some manufacturers say that they're used to keep the shoes closer to the drums (to reduce pedal travel) and others that the slight pressure prevents fluid from coming in when the cylinders retract.

I don't know. I figured I'll use a disc/drum MC just in case.

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.

 

[beardeddummy]

Knuckle, i just read the info on the residual valve and that would definitely fix the issue

 

[east302]

Knuckle,

Thanks for the clarification. That helps clear it up.


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