My son told me that the feel of the brakes on his '84 190D suddenly changed while he was driving. We bled the front brakes with no problem, but we get nothing but air out of the rear brake lines. I am using a hand operated vacuum pump and when I open the bleeder nipplesI only get air. There is no sign of brake fluid leaking anywhere, and I don't know where to go next...master cylinder? some other hydraulic component I'm not aware of? any help is appreciated. Thanks

 

Sounds like you need a master cylinder, Try bleeding the brakes manually.