'96 Jetta Brakes

I have a good one for all you Jetta people out there. I have a 96 Jetta GL 4 cylinder. I have had brake problems since I bought the car, I replaced all the front and rear (drums) pads and rotors. About a week later I got in the car and the brake pedal went straight to the floor, no brakes at all, but I could pump them up and get pressure. Since the master cylinder was suspect I changed it and all was fine for about a month or two. This morning I had the same problem, only this time I could not pump any pressure into the system as before. The car was not touched all night, all of a sudden no brakes.
Has anyone had a problem like this before, or know if VW has any odd ball parts I may be missing.
I have been workng on cars for over 20 years and have never seen any problem like this????
Any help would be greatly appreciated.

Did you check the wheel cylinders? One may have a leak or could be frozen. Has the fluid level changed?

I changed the fluid when I bought the car, which helped, and none of the cylinders are frozen.
Also the fluid level has not gone down any, so I do not suspect a leak.
I know there is a bypass valve on the rear brakes, could that cause a problem like this?
Thanks

It might, or you have a total luser who takes joy in disabling your brakes, hehehe. Got any evil neighbors? JK. Dont know what causes the problem, but it could be electrical. I would advise going to a mechanic if the prob persists, no brakes means no good.

What about the power booster. I know that this going bad could cause the break pedal to hit the floor.

Propotioning valve wouldn't cause this unless it was leaking. If you checked all the hydraulics then I agree, check the vacuum booster.

I'm Having The Same Issue And Without Hesitating Ordered A New Master Cyl. Wouldn't A Bad Vac Booster Make Idling Bad And Wouldn't You Hear The Leak(it Would Be A Big One). One Thing I Noticed Is I Can Pump The Brakes Up A Little But In A Few Seconds It Releases The Brakes.as I Hold The Pedal Down..i'm Sure I'm On The Right Track But I Only Have Spent 10 Minutes On It So Far...any Advice Adrressing This Would Be Nice...i Don't Lose Fluid ...i'm Not Going Aqround Using The Emergency Brake So I Need To Get This Fixed