1970s 76 coupe intermittant brake light

["Cadillac Kid" Greg Surfas 15364]

I seem to have a gremlin in my 76 CdV's braking system. Started with "brake" light coming on with the brake application. Brakes stop evenly, no pulling or locking up. First thing "it must be the master cylinder". Replaced the MC, no change braking seems to be a little soft. Checked front pads, more than okay.
Anyone out there have any suggestions?
Greg Surfas

[TJ Hopland]

I assume we are talking about the 'telltale' light on the dash?  I don't have 76 wiring diagrams available but that would be the first place I would look to see how many ways that light can be activated.   Is it just that single wire on the prop valve?  I think non Cads also had it tied to the parking brake.  If that is also the case with this car maybe there is something flexing letting the parking brake switch activate?

I think on this era GM the light other than possibly the parking brake is a switch on the prop valve. There is a floating piston between the front and rear sections of the valve/system.  Normally that piston stays in the center and keeps the switch/light off.  If there is an imbalance it shifts to the weak side which I think may also be part of the safety feature that it blocks that port to give you a little more pedal.  I know about that function because I have had cars where after doing brake work and in the bleeding process it gets stuck. 

The wiring diagram would also give you some clues of other systems that may be on the same fuse to check and make sure they are properly working.  Could not be brake system related at all, could be some other system has a blown fuse and things are back feeding through something like the bulb test circuit in the ignition switch. To eliminate that I guess you could just pull the wire off the prop valve and if the light stays off then its got to be the switch that is making it come on. 

["Cadillac Kid" Greg Surfas 15364]

Guess I didn't make it clear that there is a direct correlation between a slight brake fade and the warning light
Greg surfas

[TJ Hopland]

When the light first came on was there some fade or is the fade since the MC was replaced? 

Have you inspected the rears lately? Maybe they are out of adjustment for some reason or there is some issue when there is moderate braking pressure on them that something 'gives' a  bit causing an imbalance that is tripping the light?

How are the hoses?  They can do some strange things when they get some age on em.  Maybe get the back off the ground and have an assistant apply different levels of pedal pressure while you try turning the wheels?  See if they are working with lower pressure?