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my 1978 300d's brakes the pedal is firm but it still has weak stopping power ran the car pushed brake pedal and turned key off but car did not. that means a vacuum leak (right?). tested vac. line to booster, was 15. with the vac. line plugged i tried the same engine test and it passed does that mean my booster is bad?(internal leak?). also my brake light was on (not the wear light) it has fluid and parking brake was not On. upon closer inspection of this
the little thing under the rubber cover popped up when i pushed it back in the light turned off. what does that sensor/switch do?
thanks, time to go to work

It looks like a pressure-differential-sensor switch. This gets tripped if the fluid pressure differs between the two halves of the system. The higher pressure side pushes a plunger that trips the switch.

I'm not sure that you have such a switch, but it does look like that and such a switch would either be in the master cylinder or in a block that connects to both halves. These blocks are common on old American iron, and are often (and incorrectly) called a "proportioning valve."

If this is what it is, you need to check the hydraulic system for leaks. A bad master cylinder could conceivably cause this too.