[Vwdiesel] Brakes?

[Joe]

Greetings and salivations...

I've got a brake question.  82 Rabbit 1.6 n/a diesel. I was coming back from
a long weekend trip (about 600 miles round-trip) and first discovered my
brakes acting "not quite right" on one of the last rest areas on the
interstate.  The best way I can explain it is to say it's like the vacuum
assist just isn't assisting.  Damn near hit the curb as I was parking, as I
wasn't expecting it.  Later, I discovered that the braking "power" seems to
be related to engine RPM.  If the engine is running slow, I have to really
mash on the brake pedal to stop... But it does stop.  Popped the hood today,
checked all the hoses I could see and nothing appears to be loose or
damaged.  But I'm no expert.  I know this engine has a vacuum pump, which
drives the brake booster amongst other things.  So where do I start to
diagnose this?

Thanks in advance,

Joe