The filter would probably help if it hasn't been done in a while, routine maintenance. If that doesn't help, you may have to take the valve body out and clean the little strainers in it, which could be gunked up.

__________________
TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth