1993 GMT400 4WAL - Where is the residual valve?

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delta_p

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I would think it wouldn't hurt anything to add one if someone wanted. It's just a check valve that flows one way so it would reduce the slight backflow and hold the pistons out and with a little retained pressure in the line between it and the pistons. If it was on the rear line it would also allow the MC to send more take up fluid to the front until the pressure was reached to exceed and crack open the check valve and let the secondary MC piston to move. The pedal should feel a little better.
 

delta_p

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My issue is I did an ABS delete a year ago (the master, both front brake hoses and one rear brake piston were replaced at the same time), and if I really mash the pedal, I get a brake warning light. I've had it bled multiple times... As far as I can tell, the brakes themselves work fine, but the pedal is still mushy and it triggers it. What's throwing that combo valve out to the point it triggers that light?

Are you sure there isn't a slight fluid leak or the rear shoes aren't adjusted good. Anything like that may allow pressure to build on one side of the combo valve slightly ahead of the other side on hard braking and shuttle the red brake warning light.
 

Gibson

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Hipster - on pre-abs trucks, the master cyl had a little check valve in the port to hold residual pressure to keep 10psi in the rear brake lines. As I understand it, this prevented the shoes from being pulled all the way back by the springs, lessening the amount of fluid and time needed for the rear brakes to work.
The purpose of "residual" pressure, (usually ~10lbs,) is to keep the cups expanded in the wheel cylinders.
The residual pressure does nothing to keep the shoes extended.
The retraction springs are easily able to overcome the residual pressure in the lines.
 

Gibson

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With front discs operating in the 1,000 to ~1,800 psi range, (or more,) depending upon conditions, and the rear drums operating in the 400 to ~600 psi range, the idea that a 10 psi valve does anything to change rear braking, or the front/rear balance is totally false.
Their is almost zero/little braking effort in the front until the rear shoes have expanded out enough to begin some braking effort, THEN and only then will the line pressure build-up enough to start actually get much braking in the front,, and that is one of the jobs of the combination valve.
The residual valve plays no part in any of that, and it most certainly does nothing to keep the rear shoes partially expanded.
Hydraulics 101.
 

El Tigre

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So, when I remove a rear drum then crack open the bleeder. There will be no fluid expelled ,and absolutely zero movement of brake shoes via wheel cylinder contraction as stored pressure is released? That seems like what you're saying...
 

Gibson

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So, when I remove a rear drum then crack open the bleeder. There will be no fluid expelled ,and absolutely zero movement of brake shoes via wheel cylinder contraction as stored pressure is released? That seems like what you're saying...

Pretty much, you mention "absolutely zero movement", well their might be a couple thousands of movement.
The "sticky" part about understanding hydraulics is that we're all conditioned to think about pressure as something like a tire, where if you increase the pressure the tire expands.
But air compresses, and fluids don't.
A solid column of fluid, (no air,) wont matter if it has 10lbs or a 1,000lbs of "pressure" on it, when you release the pressure the fluid wont expand.
You can open the bleeder valve and release that 10lbs, but the fluid wont come squirting out, because their is no compressible air or gas above it that will expand and push the fluid out,, that's why 100% bleed jobs are so important.
If the brake line has ZERO air in it opening the bleeder should only let-out maybe a drop or two of fluid,, and that do to gravity.
Another thing is that when the pads and shoes are in full contact with the discs and drums their is NO more movement of fluid, their is only the transfer of pressure thru the fluid.
Because the pads are already starting out in contact with the discs, the only "movement" of fluid by the MC is just the amount necessary to expand the shoes into full contact with the drums.
Those videos that show the pads pushing-out to touch the discs when you push the brake pedal are false, actual pad movement is very tiny, on the order of thousands.
 

El Tigre

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We're talking about rear drum brakes, not disc. I found a significant improvement in pedal height , and feel. Obviously as brake shoes actually meet their respective drum(s) the operation/function is the same.... Perhaps that is what you meant.
 

Gibson

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We're talking about rear drum brakes, not disc. I found a significant improvement in pedal height , and feel. Obviously as brake shoes actually meet their respective drum(s) the operation/function is the same.... Perhaps that is what you meant.
Well, I mentioned the discs because of the countless posts where the posters talk about "How much fluid you have to move because the pistons in the calipers are so much bigger than the wheel cylinders".
And that is just so wrong,, but it's a thing that gets endlessly repeated.
The original purpose of residual valves come from the old days when the MC was mounted underneath the floorboard and was below the level of the wheel cylinders.
With that arrangement gravity would try to drain the fluid from the wheel cylinders back to the MC, and the wheel cylinder cups would let air leak past them into the system,, that's also where the idea of the little springs inside of the cups came from, to keep the cups expanded so as not to let in air from one side nor let fluid leak from the inside.
A residual is always a good idea, but it doesn't change or affect the return springs.
 

El Tigre

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One thing that does cause issues among folks who have late model Impala's , and/or Caprice where I have more expertise... Is the existence of master cylinders , and calipers available in different bore diameters across generations/production years. With so many more drivetrain options among trucks like ours. I wouldn't be surprised if a technically "wrong" part bolted right up with equally mysterious side effects.
 
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