Odd brake behavior W123
 

A few times recently, I've pushed the brake pedal and I felt like it was spongier than normal and travel was increased. Letting go and pressing the pedal a second time yielded a normally hard pedal. Fluid level in both ends of the tank is full. No leakage at the wheel cylinders is evident.

Am I correct that this seems like a master cylinder sealing issue and that replacing or re-sealing the MC should solve it? Or could air be getting into the system at the wheel cylinders without fluid out?

Getting this looked at by a mechanic tomorrow. Between this and the alt issues, the car seems a bit jinxed :)

Sounds like a bit of air in the system

Why/where would air suddenly enter the system?

how new are your hoses?

No idea.

Rarely, I've seen this happen with a loose wheel bearing. Otherwise, if there are no external fluid leaks, it's most likely a failing master cylinder.

Happy Motoring, Mark

Sounds like a MC. The original hoses on my 240D went bad after 30 years. Delaminated internally. It felt more like a sticking piston. The MC can push the fluid to the caliper but the fluid has a difficult time returning. Neither the master cylinder or brake hoses are difficult to do given the time, space and inclination.:)

If Air somehow did not get into the system it sounds like a Master Cylinder Problem.
I have not looked for the prices on them for a long time but at one time a Master Cylinder rebuild kit was nearly the cost of a rebuilt Master Cylinder. And, the ability to re-seal the Master Cylinder depends on how deep the Pits are in the original Master Cylinder.

Rebuilt Master Cylinders from local Part Stores do not have a good reputation but at least there is no shipping charge to argue over when need to return a bad one.
I bought one locally and out of the Box the Pedal would sink slowly. It laste 1.5 Years and the Pedal went to the Floor.

At the time the price was good on a new FENCO (Taiwan) Mastery Cylinder and it so far has given 5.5 Years of trouble free service. New Master Cylinder FENCO NM52326 (no ABS on my car), this is not the same as a FENCO Rebuilt Master Cylinder.

After installing the FENCO I found My lost Paperwork for the life time warranty on the rebuilt one and got another free rebuilt one.

Also on My Year and Model the front Piston on the Master Cylinder has a Pin that goes through it. In order to remover the Front Piston you would need to pull that Pin out.
Item 27 in the Pic is the Pin.

I don't know if internal problems in the Brake Lines can cause the issue you are speaking off. But, I think Hunter has a Post where it has that after 5 years of use Brake Lines are starting to deteriorate.

30 Year Old Brake Hoses????

I don't know if internal problems in the Brake Lines can cause the issue you are speaking off. But, I think Hunter has a Post where it has that after 5 years of use Brake Lines are starting to deteriorate.

30 Year Old Brake Hoses????

I Notice some sites including Pelican are selling New Metelli Master Cylinders at a reasonable cost. Does anyone know where they are made?

Recent intermittent excess brake pedal travel problems after half an hour or more of driving at 100 km/hr on my W123 300TD appear to have been caused by a sticking caliper piston with the fluid boiling in consequence. I finally found it by walking around the car and testing the wheel centre temperatures with the back of my hand. Because the hydraulics are split system I still had braking ( with excess travel) at the front.

Spud, my money is on MC internal piston seal. A leaky seal will allow fluid to migrate from the front to the back of the piston when under pressure. The MC depends on pressure differential between front and back to operate.

You did not state what year or model W123 you have, but the same MC
probably the same across the board?

Here is what pelican Parts shows for new MC.
I agree sounds like the MC is leaking internally, I would go with a new one.

1984 Mercedes-Benz 300D Base Sedan - Brake Hydraulics - Page 4

What does that mean ?

The air side of the piston sees atmospheric pressure whereas the fluid side sees pressure exerted by the brake pedal. If the fluid is on both sides of the piston, brake pedal pressure cannot transfer to the wheel cylinders.