Brake Problem
 

I am going down interstate at 65 to 70 for 10 or 15 miles and when I take the exix ramp my brakes for almost to the floor.
I can drive it on down the road a ways and brakes come back and are as good as ever.
Checked for leaks and found none and Brake fluid is a normal level.
This has happen 2 times now. This is on the 1984 300d

Check the vacuum. if not enough vac then maybe a leak in your vac lines system around the brakesytem. Or your vacuum pump is failing.

check your brake vacuum booster also.

do a search of archives for methods.

good luck:wacko:

also might be failing master cylinder. first stage may fail under heavy braking and then you just engage the second stage near the foorboard.

tom w

To me that sounds like your master cylinder is going. Possibly the highway trip is making the fluid warm up, soften the harden seals within the master cylinder and allow it to seep under pressure.. Thus the pedal to the floor. Better get that check right away, you don't want to mess with your life.

Sounds like a master cylinder. Don't wait or you will crash into something.

I agree. Master cylinder.

Not likely vacuum related. That would give a firm pedal that required a lot of effort to stop, rather than a pedal that goes down to the floor.

One old fashioned master cylinder test....
 

Is to press and hold the pedal, with some pretty good pressure.

If it slowly sinks any amount, no further questions, it's bad, replace it.

If your brakes are acting up, and you don't have enough experience, get some help. Life is too short for bad brakes.

I disagree with everyone. I had the exact same symptoms and it turned out to be a bad caliper. I had to emergency brake and blew the caliper, once it was replaced the disappearing reappearing brakes never happened again.

Another Master Cylinder test, have the car in gear, with enough pressure on the brake pedal to hold the car still. Do not adjust foot pressure or position. If the car begins to creep, bad Master Cylinder. A lot oftimes this is noted while sitting at a traffic light. Increase in pedal pressure or reapplication of brakes works for a minute or so.

With a bad caliper seal one should notice the loss of brake fluid. As with any leak in the breaking system.

brakes
 

a 1983 300sd has awsome emergency brakes... they are inside the hub of the rear rotor.... so apply your emergency brakes and see if the car will move forward. (in an emergency you can hold the emergency brake release and use the emergency brake to stop) Its a totally seperate system from your fluid system.


NOW, Check your whole brake system.... you could have a sticking caliper.... if your brake pads are completely gone then the caliper piston could be stuck at the end of its travel....Replace all your pads and rotors......Rotors are so cheap now I never have them turned.... I bought a rotor for 15 bucks the other day....If you have a lip on the rotor then replace it.....I recently replaced all 4 of my rotors and all pads for under 100 bucks....

Normally I dont recommend bleeding brakes but since your experienceing a mushy pedal.... (which could be caused by air) go ahead and bleed the heck out of them..... bleed all 4.... be carefule with the nipples.....If your pedal is mushy ocassionally and then not mushy... its usually air in the system... which gets in when you max out a caliper with totally worn pads..

If this does not fix your problem then buy a master cylinder.... and also replace any bad caliper..... both of these are inexpensive these days...

yet again, blueranger has nailed it.
 

I agree with him. caliper would show leakage, and master cylinder would act differantly, this is most likely either air in system, or a stuck caliper causing the fluid to boil. a fluid change and pad replacement, then good followup on pad wear should show you which caliper should need replacement if it does after pad replacement which would put the piston further in the sleeve.
Good luck,
John

Update
 

I ordered a master cly. before the pad & caliper post came up.
I will also check pads tomorrow and give and up date then Thanks for the help....Ronald

pads and rotors
 

replace the pads and rotors....and bleed the heck out of all of them...
basically change the fluid...

then test it... and then if needed change the mastercylinder..

put a little lock tite on the 17mm bolts that hold the calipers on when you take them off.... once the calipers are off I put the bolts back in the holes and use them to help push off the old rotors....be careful..

hang the claipers with wire....