As long as the reservoir is clean and the cap fits well, I wouldn't worry about it. The proportioning valve won't block the fluid flow to the rear brakes. It meters the fluid between the front and rear brakes. If for some reason the front or rear brakes take more fluid than the metered amount because of a bypassing master cylinder, or a broken hose or line, the valves goes out of it's preset range and triggers the "brake" light on the dash. Air in the brake system will have the same effect. Most times you can't pump the brake pedal fast enough to force out the air. It keeps moving up in the lines. I would recommend taking the truck in and having the system "pressure bled". This moves the fluid through the brake system fast enough to remove the air. You may find you have a problem with the master cylinder caused by the contamination, but you won't be able to tell until the air is out. Hope this helps.
Tuesday, January 13th, 2009 AT 2:56 PM