1979 300SD brake woes - PeachParts Mercedes-Benz Forum

Maximan1 · #1 08-08-2025, 05:02 PM

I picked up a 1979 300SD with a little over 90k original miles on it. A friend of mine bought it from the original owners grandson, and I bought it from my friend. It obviously has sat for a lot of time, and the brakes were not in great shape. I was trying to get it driveable for my wedding back in March, but that ended up not happening.

When I got it the front brakes had no fluid. I added fluid and discovered a leak between the caliper halves on one side and they would start to stick/seize up when driving, so I replaced the calipers with good used units. Those started to stick as well, so I replaced the master cylinder and brake hoses to the front. They still stuck, so I replaced the good used calipers with another set of good used calipers. Those stuck also, so I replaced the calipers and rotors with the early w126 vented rotors and calipers, and new hoses again.

That is where I'm at now. I still have the front passenger side caliper sticking when I drive. If I pull over and crack the bleeder, I have fluid shoot out and it drives well for 10-15 miles. I'm afraid to to father than that. 4am the night before my wedding I thought I had it fixed but after a test drive I only got a brake overheat warning light, as pictured below.

I did notice that I have a rear brake hose weeping also, but that should not have any effect on the front brake sticking or seizing. I am nearing my wits end here, I can't imagine I've had 4 bad calipers. Is there a way for the hard lines to restrict fluid flow this way? Anyone else had to deal with this before? I wanted to make this car a nice reliable driver but I've never had this many issues with brakes before.

Thanks
Max

engatwork · #2 08-08-2025, 06:29 PM

Have you replaced all the rubber brake hoses at the wheels? Did you blow out all the hard lines while system was opened up? You probably need to replace all calipers with new or good reman units along with all the rubber hoses at the wheels

Sugar Bear · #3 08-08-2025, 07:04 PM

The brakes may be binding due to the pads sticking in the calipers. Yes make sure the steel lines are clear sans old coagulated fluid, dirt and or rust.

Since fluid comes out and you've replaced hoses etc. try loosening the master from the booster to see if the brakes release. It could be an incorrect MC, pushrod too long or the booster not releasing. Does the pedal work freely with zero binding?

vstech · #4 08-08-2025, 10:00 PM

+1 on the booster being at fault.

Replacing the rear hoses can be an issue as well, as pressure in the system affects them all.
Used after used caliper makes no sense.

Either rebuild yours, or get remans. Change BOTH as a set, so you dont get uneven braking.

Maximan1 · #5 08-08-2025, 11:09 PM

Last set of calipers were reman units. I have taken the MC free from the booster and still had stuck brakes, so I don't think it's pushrod length or anything with the booster. I did replace front hoses (twice) and about to replace rear hoses as well. I did not blow out the hard lines when I had it all apart but I'm about ready to take the lines off and blow them out with compressed air since they are the only part of the front brakes that have not been replaced.

rwd4evr · #6 08-09-2025, 03:43 AM

Has to be something in the lines if it's not the master not releasing all the way. Just for a test try cracking the line at the master for the rear brakes when it starts sticking. I don't think residual pressure in the rear will stick the front but I'd love to know if it's possible.

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Diesel911 · #7 08-11-2025, 02:23 PM

Quote:

Originally Posted by engatwork

Have you replaced all the rubber brake hoses at the wheels? Did you blow out all the hard lines while system was opened up? You probably need to replace all calipers with new or good reman units along with all the rubber hoses at the wheels

This. Not normally something I would do but due to his symptoms yes.

Maximan1 · #8 08-11-2025, 09:47 PM

I'm going to clean/flush the brake lines on Wednesday, I will update this thread.

Maximan1 · #9 08-12-2025, 08:46 PM

Flushed the lines today using alcohol. Bleeding the front passenger side I had millions of micro bubbles. I did almost an entire quart of brake fluid through that side and still had microbubbles, however I think this may be from the bleeder sucking in air from around the threads. I'm not sure, I ran out of brake fluid to bleed.

It is still doing the same thing, to a lesser extent. Pass front wheel was significantly hotter than driver front wheel after about a 5 mile drive. I may replace the passenger front caliper (under warranty).

Diesel911 · #10 08-16-2025, 10:42 AM

Quote:

Originally Posted by Maximan1

Flushed the lines today using alcohol. Bleeding the front passenger side I had millions of micro bubbles. I did almost an entire quart of brake fluid through that side and still had microbubbles, however I think this may be from the bleeder sucking in air from around the threads. I'm not sure, I ran out of brake fluid to bleed.

It is still doing the same thing, to a lesser extent. Pass front wheel was significantly hotter than driver front wheel after about a 5 mile drive. I may replace the passenger front caliper (under warranty).

Off subject maybe. Since the 1960's I had always adjusted my front wheel hub end play by hand. When I did that on the Mercedes it resulted in the me getting the clearance too tight and I roasted all of the new grease I packed into the hub and had to clean that all out and regrease. Fortunately, I had the sense to drive it a few blocks and check for the hub getting hot.

After that I started using the Mercedes service manual method using a dial indicator and sins that time in 2008 have had zero issues.

To see if the brakes are dragging jack the front wheels safely up off of the ground. Start the engine up and depress the brakes hard several times. Go out and pic one front tire and rotate it by hand. There should be some intimal drag but after that it should free up with only some tiny amount of drag.

Repeat the break depression and do the same to the other tire.

Air trapped in the lines can also cause the pads to drag because after you release the brake pedal the air in that part of the system is still compressed keeping the pads from returning as they should.