Brake Light on dash, 1967 Impala

About 10 years go, one of the first items I did was switched to 69 Front disks brakes, used all the components from the 69 out side what was the "available " cure for new rotors, not long after that switched to tach dash with ALL new harnesses from AAW.
Since I have had this issue w/the car where the brake light would not come on when I used the parking brake, which was not a big deal until I switched from automatic to 4 speed.
Then last year I took a trip from Phoenix to Pomona swap meet, and the light started to flicker, then out of nowhere, the light started to work when I used the parking brake, and when I was not using parking brake, it at times it would flicker, long story short, it works real bright using parking brake, and now it is on full time but not bright.
So last weekend I put in a new master cylinder and "hold off valve" (not sure what that valve is), but replaced it since I was going to be there, found an issue with my booster, fixed it, bench bled the master cylinder before install, bled all brakes, light went out woo hoo I thought, last night, I start the car up and the red light is on dim again after releasing the parking brake, good pedal, no leaks.

I am planning a 1100 mile (one way trip !) to Spearfish SD next week (leaving Friday or Saturday) will be going through some mountains so brakes is something I don't want to scrimp on!

Going back to the "hold off valve" is there something I should know about this thing? Looking for some guidance, brakes seem to work good ideas?

I am not a mechanic, hate grease but can do most work myself outside a motor rebuild or trans rebuild, body was my field of choice, and in the 11-12 years or so I have owned it, the only thing I have done "body" on this car is paint the black on the rear body panel and the front argent filler.
The rest is for future Clone car, Paint & Body then.