CDR testing can be accomplished with 4' section of clear plastic tubing, sized to fit the dipstick tube.
Form a 3" U-bend , with 12" vertical columns. This could be attached to a piece of 1 x 6 with large u-staples, parallel-spaced at one inch intervals for indication of measurement.
Important - do not crush or deform the plastic tubing.
Place the free end over the dipstick tube, ensuring tight fit, with no leak-by. Position the manometer vertically, where the upper end of the U is lower than the end over the dipstick tube.
Note: this is to prevent water siphoning into the oil pan thru the dipstick tube.
Pour colored water into the U section such that about 6 " of water is in each leg, about 4 - 6" below the top of the open end.
Have someone start the engine, bring the rpm up to about 2000, keeping close watch on the water level in the U-tube. If the water in the dipstick leg rises an inch, the water in the open leg will drop an equal amount. Adding the drop level to the rise level, this would indicate vacuum at two inches on the Water Column.
Vacuum at idle should be around 1", depending on air filter restriction; at 2000 rpm, the CDR should limit vacuum to 4 - 6" WC on a calibrated Manometer. The shade-tree version should indicate close to those figures, depending on accuracy of staple intervals.
If the level in the dipstick leg drops, and the open leg level rises, this indicates the crankcase is slightly pressurized.
27" WC is equal to 1 PSI (Pound per Square Inch) 1psi is equal to 2" mercury
Atmospheric pressure is 14.7 psi, or 29" mercury, or 396" WC, rounding off
The CDR should never be solvent-cleaned, as various solvents can dissolve the diaphragm, again resulting in a 'no-way' valve.
Some truck manuals indicate replacment at 30,000 miles. This refers to the early style CDR and oil-cap breather system, used up through mid-'80s. By '88 - '89, the CDR system had been revised to its present configuration, where the CDR cannister is on the passenger-side valve cover, with one external connecting hose to the intake system.
Early failure-prone systems can be easily upgraded to the late setup - 1 valve cover with grommet seal, 1 CDR, 1 hose.
Replace the CDR in the late systems when it fails the Manometer water column test.