I've got a 19mm master cylinder. The brake light blinks while I'm driving. It goes off if I raise the e-brake switch.

Any suggestions?

That is odd. Do you mean the light in the gauge? the reason it is odd is because the circuit gets its ground when the break lever is up which allows the contacts to touch inside the switch. When you put the lever down it breaks contact.

You could unscrew the single retaining screw for the switch under the lever and pull the switch out. Maybe something is grounding it out when the lever is down and the contacts are too dirty to work when it is up? When you have he switch out you can see what the deal is. If when it is out (not touching metal) it still causes blinking then your problem is somewhere else (I hope not).

Sometimes the Emeg. Brake handle will be bent a little and misss the contact swith to turn off the light. Just bent it to make sure it is pushing the contact down.
Van

Would it blink with Parking brake on?
I know it blinks with problem in hydraulic system. Forgot to reset it this morning after a brake bleed. That's how I know.
Never had a 914 with working Parking brake switch

Both...it blinks with the brake on and when the MC switch needs a reset after a bleed.

Sounds like the switch is having issues and thinks the e-brake is on.

If none of the above works try tracking the brown wire that feeds the switch. It routes behind the seat and into the center tunnel. Look for a pinch point or frayed insulator