my friend had a 80 vette and the brake light comes on. sometimes when the brakes are pumped the light will go off. It has new calipers, new master cylinder. The brakes have been blead but after a while the light will come on and I have to pump the brakes What do you think you think the problem could be? 

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The switch at the brake pedal could be loose or have a loose connection. Do both brake lights come on or just one? If only one then that socket may have a loose connection, if both come on, then I would check the switch.

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Lifetime Member #116
Dave's '82

   

its not the rear brake light in the rear, its the light on the dash that tells you that you have a brake problem.

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the pedel goes down and I have to pump the brakes back up. new master cylinder, new calipers, and bled the brakes, 

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You may still have air in there somewhere.

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Lifetime Member #116
Dave's '82

   

Hey Frank,

I agree with Dave. Could still be some air in the system.

I always have best luck using a pressure bleeder.  There was a posting here a couple of years ago showing how to make a cheap one.

Larry

And if all else fails, there is still the possibility of the proportioning valve.

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Jimmy B.
Just can't wait to get on the road again.

 
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I have bled brakes the traditional two person way and have a pressure bleeder as well but the quickest and easiest bleeding Ive done is by using "speedbleeders".  They are cheap, work well and easy to use by one person. 

If the proportioning valve was bad the light would stay on, it sets itself if their is unequal pressure between front and rear brakes, encase of a bad leak so as to only loose one set front or rear. If breaks bleed down and can be pump back up I believe it sounds more like the mastercylender you can try pumping up the breaks and on the last pump hold and if breaks bleed down it would be mastercylender