1982 300D Turbo 366,000 miles. Brakes sometimes fail after driving at high speeds for 1/2 hour or so. 70mph. By failing mean very long pedal and weak braking power. Replaced front drivers side caliper this was defiitly sticking rubber was melted about 8k ago bled brakes 1/2 dozen times. Put 10,000 miles on in last 2 months. Take 1000 mile trips no issues. Noticed noise from front driver rotor yesterday and brakes fading badly. No fun coming up on a slow tractor trailer at 70mph and no brakes. Just ordered front brake hose will change today, been getting all this free advice from friends like change master cylinder and see if that does it. To find out if it worked I will have no brakes at 75mph someday and try something else. Is the story of hoses swelling with age age keeping pads locked on the rotors true? Maybe most hoses have been changed out 10 years ago. Don't hear this anymore. Car loves to run stopped using oil and runs faster and smoother after runnning the hell out of it.

 

More likely a combination of warped rotors and sticking calipers. The heat from the constant friction causes the fade. Does the car shudder when you apply the brakes?

And yes, it is true that brake hoses fail with age causing a constriction inside the line. However, what is often the case is that you get an oxidation buildup behind the piston seal in the caliper that squeezes the seal against the piston effectively locking it in place. A rebuild kit is usually about $40. as opposed to $200 for a new caliper. When I got my car all 4 rotors were warped and the inside piston on the two rear calipers was sticking. On the right one I was able to free the piston and rebuild it, but the left had to be replaced.

 

I've had a similar problem, not sure what's up yet: on two different occasions with two different 300Ds (one a hot day driving across town in traffic), brakes working fine, but suddenly find the pedal going to the floor with no power. Pumping brake builds up pressure and car stops, but then 30 seconds later pressure is down. No visible leaks. Acts exactly like failing master cylinder: pump and hold brake pedal, and it very slowly sinks toward the floor...

however, thereafter it behaves perfectly fine, in similar hot weather and more extensive driving conditions: and I can't reproduce the slowly-sinking brake pedal. No sign of braking problems.

Are we talking about the same effect? If so, what is/isn't the problem?

 

Problem is pads rubbing on disks while driving builds up heat and causes failure. Either you need to rebuild calipers, replace rotors, replace hoses, flush fluid or some combination of all four.

 

many thanks for sharing your expertise! 's amazing, after some years of doing my own maintenance and repair, how many critical details there are to learn.