disc brake ?

Do you mean your brake lights or the small dash brake warning light that's hooked to the proportioning valve (P-valve)?

You said you replaced the proportioning valve with the correct new master.....but that doesn't make any sense. Those are two seperate items. Do you have a proportioning valve at all?

If it's the warning light then you must make sure you have all the lines running to your P-valve exactly how a shop manual says you should, also make sure the P-valve is working correctly and you have proper line pressure. It might just not be working right. Personally I'd scrap the factory P-valve and go with an aftermarket Wilwood or Mopar Performance adjustable P-valve without the light.

The light is just there to tell if you you've blown a brake line or have a failure somewhere........and I don't know about you, but I know when I've blown a brake line! I don't need a dummy light to tell me that!