'78 300 D brake issues

[Bierjager]

Howdy all,
I'm reletavely new here and I am having an issue with my brakes.
I was driving the car home tonght from work and I noticed that the brakes were taking more pedal travel than usual to stop the car.
It kept getting worse the closer I got to home.
By the time I pulled into the driveway I only had rear brakes.
I opened the hood expecting to see a near empty resivoir, but it was nearly full still.
I peeked under the car expecting to a leaking hose or caliper or something, dry as a bone.
Same thing with the rears.
I got back in the car and I found that I could pump the pedal and get better pressure.
I left it alone for a bit and tried the pedal again and it seemed to stay up.
It's acting really wierd.
I'm wondering if one of the seals in the master cylinder took a dump and isn't generating any pressure in the front brake circuit.
Is this possible with these cars?
I'm new to Mercedes Benzes but not to older cars.
Usually when I have an issue like this there's an obvious reason, like a leaky hose or something.
Am I missing something?
Help!!!