Sounds like your modulator valve is either bad or not getting vacuum.
And I am sure that you are keeping the fluid at the right level. If it gets to low it will do that.
1985 500SEL (260,000 miles) SOLD!
1974 280 SOLD!
1994 Volvo 850 Turbo Wagon (Yuppie Ice Sled)
"I know I have a Benz and they know it too." If you start bragging about it or shoving in people's face, you then become a Wally; especially to the girls.