Brake fluid not reaching back brakes

RACHELMART19

MEMBER
2003 CHEVROLET TAHOE
V8
4WD
AUTOMATIC
102,000 MILES

My brake fluid/system was ruined due to a faulty resevoir cap and condensation build up. The brake fluid was drained yet the rear fluid would not drain, since then the master cylinder(refurb) has been replaced yet the brake fluid is still not reaching the rear. I've done some research and I've been told it is the air lock in the proportion valve. Does that sound right and what is that? How would you replace? Also the resevoir had a green build up around it it has since been cleaned- Do you recommended replacing that altogether?

1 Reply

JAMES W.

MECHANIC
2,395 POSTS

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. Here is a video and guide below to help you bleed the brake system correctly.

https://youtu.be/w7gUsj2us0U

and

https://www.2carpros.com/articles/how-to-bleed-or-flush-a-car-brake-system

and

https://youtu.be/WDxvEQrMkBg

Please run down these guides and report back.
Hope this helps.