1975 Chevy K20 Brakes will not bleed

[wazzadakka94]

I have a 1975 Chevy K20 I have replaced almost everything in the brakes system minus the hard lines and the proportioning valve. I have kept the master cylinder full the whole time, yet I keep hard air in the front brakes for some reason. I have check every single connection to make sure that it is tight. I have been using a vaccuum pump to pull the air out of the front brake lines, yet even after sitting for 30 minutes their is still air in the system someone how.

What can be done to correct this issue?

 

[bucket]

Did you bench bleed the master cylinder before installing it?

 

[wazzadakka94]

Yes I did. My bad I forgot to add that.

 

[bucket]

It might be easier with a helper. Pump the pedal and bleed the lines at the master cylinder. Then bleed at the calipers.

 

[wazzadakka94]

I have done that and there was still air in the front brake lines I switched over to the vacuum pump to attempt to get the job finished up quicker.

 

[Dave M]

I found I had to use one of these when I did mine.

Use this Proportion Valve Bleeding Tool when bleeding your brakes to lock the prop valve in place and keep the valve from tripping of center.
you simply remove the warning light switch (white plug) on the valve, then screw in this tool while bleeding your brakes.
After the bleeding process is finished replace the warning light switch.

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[Rusty Nail]

Yep.
Lookin at you proportioning valve!

Look into proportioning valve RESET.
That's what is wrong..
Hope this helps!