grazbag

Hi There,
Sorry if someone has posted similar but I really need your help. I have bought a W123 and use it as my daily driver. In the last month it has not missed a beat over the 500 miles I have driven despite having been standing for a while prior to me buying it.
The ONLY fault is that on two occasions the brake pedal and lost it firmness and been very soft at first travel then harder at the base of the pedal travel. The car still stopped but was a bit scary. This fault is very infrequant having occured twice and then next time of using the car the pedal is back to perfect! There is no loss of fluid and I have mechanic happy to help fix the problem but I dont want to start buying parts that arent the right parts.
Thanks in advance for any assistance

Outre

Assuming you have the proper amount of Brake Fluid in it on the surface it sounds like the Master Cylinder is failing.
Also that since you bought it you did not let the Fluid get low enough that Air got into the Master Cylinder.

If it was a Vacuum Leak issue or a Power Brake Booster issue it would take a lot of effort to depress the Pedal.

However, there is an area that can effects both. If the rear Seal on the Master Cylinder leaks (this is an exterior leak that you would not see) Brake Fluid inside of the Power Brake Booster or the O-ring that seals the Vacuum at the rear of the Master Cylinder is not sealing well they symptoms might not be as clear cut.

man1nvan

the way you describe
"very soft at first travel then harder at the base of the pedal travel"
sounds very much like an issue i had, i put up with it for years, brakes would be fine for months the suddenly exactly what you describe, no servo and then they would go back to normal.

it may not be very scientific, but being as it was my brakes I replaced the cylinder and servo just to be on the safe side, anything else and i would have had a stab at it.

scoot

This happened in my 114. I'm voting for master cylinder.

grazbag

I guys, thank you for your advice and i have ordered a new master cylinder to fit at the weekend. Is it worth also repalcing the slave cylinder as well at the same time? I am struggling to source one at present.

Thanks in advance

Outre

Is the "slave cylinder" used to operate the Clutch?

If so the only relationship it has to the Master Cylinder is it gets brake fluid from one of the Brake Fluid reservoirs.
Replacing it is a separate decision.

grazbag

Guys,

I fitted my new Master Cyclinder and new 4 dot brake fluid yesterday and bed the system on all for corners through the bleed nipples. Today I felt chuffed that the problem was fixed but on the way home tonight the pedal dropped to the floor again and went spongy, only giving me braking in the last bit of the pedal. The car still stops but I just get no feel or resistance until the bottom of the pedal movemnt then braking is pretty harsh. Still no leaks in the system and I really need your help with other ideas as £70 and a sore back has not solved the issue.

Thanks again in advance for your help
Graeme

Outre

The rebuilt Master Cylinder I bought was never right and lasted about 1.5 years. I put on a brand new one and have had no issues since then.
So it is possible that you just got a bad one.

The Master Cylinder itself needs to be bleed before connecting it to the brake Lines. I they US Mastercylinders commonly come with a kit that screws into the Master Cylinder out let ports and has Plastic Tubes that you can loop the Plastic Tubes back into the Brake Fluid Container so you can bleed the Master Cylinder.

After that is done it is onto bleeding the Calipers.

If that does not work I am at a loss for ideas. When the Brake Booster is bad or the Vacuum to it is not enough the effort to brake shoots up but the Pedal does not go to the Floor.

Also I have been assuming that you donot have Anti-lock Brakes.

A spoungy feeling at the pedal is often Air still in the System.

Outre

I wanted to comment on some other problems.

If your Brake Caliper Pistons are sticking some of the Fluid does not return to the Master Cylinder as it is supposed to. I believe that can cause you to have a low pedal.
To find out if the Caliper Pistons are sticking use what ever measures needed to safely Jack up as many Wheels as you plan to check start the Engine (keep it in Park).
Have someone step down hard on the Brake Pedal. After you tell them to release the Brakes start rotating the Wheel and see how much drag there is and how long the drag is present.
As soon as the Brake is released and you start rotating the Wheel you should feel it quickly free up and the Wheel should rotate easily.
The above is more difficult to do on the Rear because you do not get the same feel for it due to you Turing the Axle and some of the differential parts. For the Rears it is best to have both Wheels off of the Ground.
The Piston can be sticking due to the Rust and muck inside of the Brake Caliper or because the inside of the Brake Hose has collapsed and is acting like a one way valve.
To check for the one way valve thing; have someone step on the Brakes and release them. After the Brake is released loosen the Brake Bleeder Screw.
If fluid spurts out It could be the Brake Hose as described or an Air Bubble in the System.

Stuck Caliper Pistons cause the Brake Pads to Drag on the Rotor and create heat. If the Heat is absorbed into the Brake Fluid it can cause problems with the Master Cylinder.