General Motors — To drain the LPR use the following steps:

A General Motors service procedure — check it applies to your exact model before use.

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Procedure

1. Move the equipment to a well ventilated area and ensure no external ignition sources are present.
2. Start the engine.
3. With the engine running close the manual valve.
4. When the engine runs out of fuel turn OFF the key when the engine stops and disconnect the battery negative cable. IMPORTANT: A small amount of fuel may still be present in the fuel line, use gloves to prevent burns, wear proper eye protection. If liquid fuels continues to flow from the connections when loosened check to make sure the manual valve is fully closed.
5. Slowly loosen the inlet fitting and disconnect.
6. Loosen the hose clamp at the outlet hose fitting and remove the hose.
7. Remove and retain the locking pin in the outlet fitting and remove the outlet fitting from the LPR
8. Disconnect PTV connection and disconnect the vacuum hose.
9. Remove the two LPR mounting bolts and retain.
10. Place a small receptacle in the engine compartment.
11. Rotate the LPR to 90° so that the outlet fitting is pointing down into the receptacle and drain the LPR.
12. Inspect the secondary chamber for any large dried particles and remove.
13. Remove the receptacle and reinstall the LPR with the two retaining bolts and tighten to specifications.
14. Reinstall the outlet fitting and secure with the previously removed locking pin.
15. Reconnect the PTV electrical connection push connector until lock “Click”, pull on the connector to ensure it is locked, connect the vacuum line.
16. Reconnect the outlet hose and secure the hose clamp.
17. Reinstall the fuel inlet line and tighten connection to specification.
18. Slowly open the manual service valve. IMPORTANT: The fuel cylinder manual valve contains an “Excess Flow Check Valve” open the manual valve slowly to prevent activating the “Excess Flow Check Valve”.
19. Check for leaks at the inlet and outlet fittings using a soapy solution or an electronic leak detector, if leaks are detected make repairs. Check coolant line connections to ensure no leaks are present.
20. Start engine recheck for leaks at the regulator.
21. Dispose of any drained material in safe and proper manner. AIR FUEL MIXER/THROTTLE CONTROL DEVICE MAINTENANCE AND INSPECTION IMPORTANT: The Air Fuel Mixer components have been specifically designed and calibrated to meet the fuel system requirements of the emission certified engine. The mixer should not be disassembled or rebuilt. If the mixer fails to operate or develops a leak the mixer should be replaced with the OEM recommended replacement parts. When inspecting the mixer check for the following items: • Check for any fuel leaks at the inlet fitting. • Check the fuel inlet hose for cracking, splitting or chaffing, replace if any of these condition exist. • Check to ensure the mixer is securely mounted. • Check air inlet hose connection and insure clamp is tight, check inlet hose for cracking, splitting or chaffing, replace if any of these condition exist. • Check air cleaner element according to the Recommended Maintenance Schedule found in this section. • Check fuel line to Throttle body mounted Fuel Trim Valve (FTV) for cracking, splitting or chaffing, replace if any of these condition exist. • Check Throttle body return action to ensure throttle shaft is not sticking repair if necessary. • Check FTV electrical connection to ensure connector is fully seated and locked. • Check for leaks at the throttle body and intake manifold. • Check Throttle cable for damage, rubbing, and kinking and free movement repair if necessary.

Source: manual:OM56041564 - GM Maintenance Manual.pdf. Figures and steps are as documented; always cross-check against the official manual and any later revisions for your exact serial number before servicing.