The Hayes ABS motors are notorious for holding onto air.
The 2000 and 2005 service manuals say to use a pressure bleeder at 20PSI. You can build one fairly cheap or buy a Motive Products brake pressure pot for $50.
Even the expensive aluminum master cylinder cap from Motive isn't as good as the GM J tool because it doesn't have a plug in the center with an O-ring that seals the cap to the master cylinder. You can find the J-35589 GM pressure bleeder master cylinder cap on Evilbay all the time. I paid $15 for mine.
J-35589 GM quarter turn pressure bleeder master cylinder cap.
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1959 M35A2 LDT465-1D SOLD
1967 Dodge W200 B383, NP420/NP201 SOLD
1969 Dodge Polara 500 B383, A833 SOLD
1972 Ford F250 FE390, NP435/NP205 SOLD
1976 Chevy K20, 6.5L, NV4500/NP208 SOLD
1986 M1008 CUCV SOLD
2000 GMC C2500, TD6.5L, NV4500
2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263
2009 Impala SS LS4 V8
RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...
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And please let us know if and how your repairs were successful.