problem with breaking on 1990 Mercedes-Benz 300TE 4MATIC

when I press the break it goes sponggie when stopping and when I pump the break it goes back to "normal".This is happening every now and again. Any suggestions?
thanks

Get Your Car Fixed at a RepairPal Certified Shop.
We certify shops based on expertise, quality work, and verified customer reviews
1 answer
Have checked at qualified shop asap. You could have master cylinder getting ready to go completely out or air in system.