There are two tools that might help
I'm using nomenclature, in quotes, from the cutaway pic in post #8...
Kent Moore J-23709 is little more than a 90 degree bent piece of stiff metal with a slot that fits under the mounting bolt. The 90 degree tang depresses and holds the "bleeder rod" to center the "pressure differential valve" piston.
There's another slick little tool that temporarily replaces the warning lamp switch and locks said piston once it's centered. This tool from Brothers Truck Parts --->
http://www.brotherstrucks.com/prodin...number=PVBT087 There are many other suppliers.
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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...
Please include at least the year and model in your threads. It'll be easier to answer your questions.
And please let us know if and how your repairs were successful.