This is on my 1980 240D...

Pedal has been squishy for a while, then the clutch started to get short (pedal had to be almost all the way to catch). I can pump the brakes and get some pressure back in the clutch for awhile

Recently the brake warning light would come on if the brake pedal was pressed and held all the way down. Now the light is on more frequently.

So I figure I need new pads, master cylinder, and likely rotors.
But how do I know if I need a new booster, too? If I am going to replace it, now would seem to be the time!

What is a diagnostic for the booster?

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Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT