Hi, the brakes on my 1984 300D seem fine when I'm just driving around town, but the other day I drove out about 20 miles (I live on Oahu and we don't have very far to drive usually) and once I got off the freeway and had to stop at a light my brake pedal went mushily almost all the way down. With a little pumping it came back up, but I had to nurse it carefully to my destination to avoid any kind of need for a sudden stop. I drove home later in the day - the same route, but didn't notice the problem when I got off the freeway at my end. Had the brakes done a few years, and very few ~10,000 miles, ago.

Anyone had this? Know what the problem is?

Many thanks in advance.

Could be a caliper sticking slightly and heating up, causing the brake fluid to boil. You would see this after long drives, but not around town.

__________________
Milan Brown 1979 240D, rebuilt OM617.952 turbo diesel, rebuilt 722.315 transmission (my only daily driver)

Instagram: @maximed93

Old fluid that has moisture in it or air in it can cause that type of behavior as well.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

How old is the brake master cylinder? Check the fluid level, if it is going down. Put a new BMC on order.

__________________
Nosce Te Ipsum "Know thyself"

The Master cylinder can leak internally. I had the same thing happen on the freeway at speed. I switched it to a known good one and all went well, no more issue. if it happens once, it will again.

__________________

Chris
84 280sl
82 300d euro

Exactly right. Water in the fluid boils when the fluid is hot. Water vapor compresses, so no brakes. Once the fluid cools, water condenses, liquid water is incompressible, so brakes return. The first thing to do is get the brake system flushed.

If he flushes he needs to do it while the brakes are hot because the bleed screw is at the top and water does not easily get up top if it is condensed at the bottom. It would also work better if the Caliper Pistons were pushed all the way in allowing less space for trapped water.

But, the sticky caliper piston/s and Master Cylinder with internal leakage are also possible. But, one would think that if it was the Master Cylinder the pedal would sink ever if very slowly.

__________________
84 300D, 82 Volvo 244Gl Diesel

bleed your brakes/flush the master with new fluid. do a visual inspection of your yourlines as best as you can. hard and soft.

You should connect with these guys, they are in HI

Renntag Post 3 https://www.peachparts.com/shopforum/diesel-discussion/351054-w126-rear-window-glass.html

Coasttocoast post 1 https://www.peachparts.com/shopforum/vintage-mercedes-forum/378762-69-230-build-w-lq4-swap.html

I had this problem with my old W123. Replaced the master cylinder and the problem was still there. Turned out to be a slightly stuck caliper on one of the front wheels.

__________________
'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

If you sit with the brakes on does the pedal go down slowly? If so it may be leaking internally.

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Is the pedal firm when you first start driving, but becomes soft after a longer drive? And is then OK again after the car sits undriven? If so, it is likely that the brake fluid is boiling due to a dragging brake, causing the soft pedal and long travel.

I can think of three reasons why a brake might drag on a 123 chassis. First, the master cylinder can fail in such a way that the rear brakes remain applied when you release the brake pedal. There is an internal piston in the master cylinder which can stick, holding the rear brakes on. The front brakes are operated by a different piston in the master cylinder, so they still function. Replacing the master cylinder is the only way to fix this problem.

Second possible cause is the flexible brake lines running from the chassis of the car to the brake caliper. These can fail internally and act as one way valves, retaining some brake line pressure at that corner of the car. I have found this problem on cars once the reach 10-12 years of age.

Finally, the brake caliper piston(s) themselves could be sticking. Replacing or rebuilding the caliper is the solution here. Note the 123 uses dual piston calipers at all four wheels, so there is no possibility that a sliding caliper is stuck, it’s always the pistons on a 123.

Here’s what I would do to diagnose it. Drive the car until the pedal starts to become soft. Then pull over and park in a safe place. Get out and walk around the car. Feel each wheel/corner of the car to see if one is radiating significantly more heat than the others. Bonus points if you have one of those infrared thermometer guns to post actual temperatures!

If only one corner seems hotter than the others, replace the brake hose, caliper, and pads on that corner, bleed the brakes, and see how it goes. If you can’t tell a difference between corners, I would suggest replacing the master cylinder because probably it is stuck and both rear brakes are applied. If you don’t know how old the flexible brake lines are, I would also suggest replacing all four of them. Always remember that brakes are the main safety system in the car! Good luck.

Agree. The first thing you need to do is drive around and then feel your wheels to see which one is much hotter than the other three. Your problem is there.

I had this and it was a sticky caliper. It is very easy to replace a caliper.

Happened to me a few times. New Master Cylinder fixed it.