Hello, My brakes on my 96 B1500 have been crappy for some time now. It will stop but the brake pedal has squish in it when pressed. So this weekend I thought I'd swap in a new master cylinder from napa as the previous one was from there. Yea, limited lifetime warranty. So after the install. Tested brakes and still the same squish. Well, as I was starring at the master cylinder reservoir while my helper was pushing the brake pedal. I noticed a disturbance in the fluid in the front chamber of the reservoir. So I took the front lid off and had my helper push the pedal again as I looked inside. When pushing the pedal, fluid/bubbles come up from the small port in the front chamber. This isn't supposed to happen I assume. Does this mean the master cylinder is bad, leaking internally? Thanks for your replies.

 

Did you bench bleed the new master cylinder before installing it?

 

Of course. bench bleeding it has nothing to do with what its doing. When you press the brake pedal, I would think the fluid is supposed to be being pushed out the metal lines from the master cylinder. Not up into the reservoir.

 

Boing...wrongo. If you do not first bench bleed then the cylinders/chambers are still filled with air and you get air bubbles.

 

Its apparent that you guys are not understanding my post. Yes, everything was bled as it should be starting w/bench bleeding.

Please correct me if I'm wrong. When a brake pedal is pushed, fluid is supposed to be sucked from the reservoir and pushed out through the metal lines in theory. Not pushed into the reservoir.

When your bench bleeding, fluid is pushed out bleeder tubes back into reservoir.

Now with this said. Should fluid be flowing into reservoir from the small port that the reservoir sits on top of when the brake pedal is pushed?

 

Yes it does.

 

If any of the brake lines have pin-holes or un-sealed connections, then bleeding could cause air to be drawn into the system. The h-block or proportion-valve could be ab issues as well.