ABS combination valve

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Ivymike

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Hi all. Both front calipers are dragging in my Tahoe. It’s JB5. They are new(rebuilt). Shop that it’s at thinks abs combination valve is restricted. Is that a possibility? When they pump the pedal up and release it, then crack a fitting between caliper and combo valve, it is still pressurized. Crack fitting between MC and combo valve there is no pressure.
The flex hoses, MC and steel lines are new also. ABS light is on. Thanks.
 
fancyTBI

fancyTBI

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What year Tahoe? The ABS light is on - pull the ABS codes and tell us what they are. Doubt the combination valve is bad. Bet there is air in the system that is trapped in the ABS.

You replaced a lot of parts so I’d place money on there being air in the ABS system. Combination valves can fail, but I don’t think it’s all that common. If it is bad you will need to source a combination valve with the same code that yours has. They are vehicle/ABS configuration specific; at least the later ‘95 up 4WAL style is.
 
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Ivymike

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It’s a ‘99 Tahoe 4WD 2dr. I feel the pedal is ok by these trucks standards. The dragging just started a month ago. There are codes. I’ll try to find them but they were related to the wheel sensors at least. Estimate was quite expensive to have repaired. So I didn’t
 
Schurkey

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Ivymike said:
Both front calipers are dragging in my Tahoe. It’s JB5.
Click to expand...
JB5 uses low-drag calipers as original equipment.

Ivymike said:
They are new(rebuilt).
Click to expand...
That's a good sign--usually.

Ivymike said:
Shop that it’s at thinks abs combination valve is restricted. Is that a possibility?
Click to expand...
Anything is possible. Not real common, though.

Ivymike said:
When they pump the pedal up and release it, then crack a fitting between caliper and combo valve, it is still pressurized. Crack fitting between MC and combo valve there is no pressure.
Click to expand...
"I" would push the metering valve stem on the combination valve using either the Genuine GM "special tool" J-39177 or aftermarket equivalent, or a home-made equivalent.

If the retained pressure vanishes and the calipers don't drag any more, you need another combination valve. (Unless you can free-up the metering valve stem, perhaps by flushing the brake fluid.)

If that doesn't fix it, it may be an ABS problem.
 
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Ivymike

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Thank you for the responses. I can’t even find a combination value available for purchase. Which obviously complicates things. I spoke to Rapid Transit Services in TX about rebuilding the abs but they said it won’t fix a hydraulic problem. I’ll look into finding the special tool.
 
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