There might be a residual pressure valve in the rear fluid circuit. I say "might" because I'm not certain; I'm just trying to give you a lead.
Take a look in the service manual, that might be a benefit. Manuals for 1998 and select other years are available in .pdf form online here and elsewhere.
GMT400 CK Service Manual Project
GMT400 CK Manual Project The 1988 - 1998 GMT400 Chevy & GMC Pickups Message Board
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Update: I looked through the 1998 manual (Section 5 - Brakes) and saw no reference to a residual pressure valve.
There's a "combination valve" in the brake circuit. With that thought in mind, check out this thread on GMT400 and note discussion about drum -> disc conversion.
If we change our obs trucks to rear disc. Mine is a 1997 Tahoe .So it came with front disc and rear drum brakes. So if I add rear disc brakes what proportion valve should I use and keep abs. Thanks in Advance
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