How difficult is to replace the master cylinder on a 1979 240D?
 

I'm at the point where I need to replace the master cylinder. I was able to locate a Mantelli one for about $50, now I need to establish whether this is something that can be done easily (DIY style, basically on a driveway with no specialized tools) or if it is much more complicated and demanding than that.
I'd much appreciate some input from those of you who have done this job.

suck contents out of resorvor, unbolt hard lines (3 I believe) unbolt mc from booster, remove tank from existing mc, press into new mc, reverse process, bleed brakes starting from rr then lr then rf and lf.

Pretty simple.

The brake master cylinders I've done have all been really easy.

Use Flair nut wrenches on the brake lines to loosen them.

Two 13mm nuts holds the MC to the Brake Booster.

Be sure to Bench bleed (clamp MC in a Vice by the ears where the bolts go through, not the body) the MC, some times there is a kit that comes with the new MC. has a couple hoses that screw into the MC. curve the hoses up and into the reservoir and be sure they are under the Brake Fluid.

Push in on the end of the MC with a blunt rod to push in the piston several times to purge out the air. when you see no more bubbles, unscrew the hoses and mount onto the Brake Booster.

Bleed the brake system as tom said. R/R, L/R, R/F and L/F caliper.

use a Pressure Bleeder, makes it so easy.


Charlie

B^^THIS

and


^^ THIS

you might get real lucky and not need to do either, but, you will save a lot of potential frustration if you do both.

Great! Thanks a lot... :)

Finally got to go through the replacement procedure..
 

The Metelli brake master cylinder did not come with a kit and could not locate one anywhere, so I improvised a bit using some 1/4" vinyl clear tubing (which by the way screws in tightly into the master cylinder) - the bench bleed procedure went through perfectly this way. Now, I have to find the time to bleed the brake system at the four calipers. I'll report results later.
Here are a couple of pictures of how I bench bled the master cylinder without a kit:

Attachment 115926

Attachment 115927

I've installed a number of master cylinders after bench bleeding without then having to bleed the brakes on the car.

yup.
the trick is to let the fluid trickle into the lines before you tighten them up to the MC.
if you allow the air to bubble out of the lines before you tighten them to the MC, you will not need to bleed the entire brake system.
HOWEVER... if the MC is bad, odds are good the fluid is bad in the car anyway, so do yourself a favor and FLUSH the entire brake system with new fluid, and prevent a seized caliper!!!

Ditto on always "bench bleed" the MC before connecting the tubes. Otherwise, you will never get all the air out. You don't have to clamp in a vise, I find it easier to pump it on the car using the pedal. True also that you don't need to bleed the whole system if just changing the MC. However, you need to replace the brake fluid every 2 years regardless to avoid severe corrosion. Actually, I don't since I use DOT 5 (silicone fluid) in all my older cars.

I changed the MC in both my 300D's in the last few years, and probably won't ever need to again w/ DOT 5. The MC cost $45 at Autozone and had to re-use my reservoirs. Many times, a failed MC will leak brake fluid into the vacuum booster. If so, remove it and try to get it out. I stuffed rags in to absorb it, then de-rusted and repainted since normal brake fluid (glycol) lifts the paint. Note also that where the MC bolts to the booster is a vacuum seal in my 84 & 85. If true, insure the O-ring is thick enough and the surfaces are clean. In most older cars, the booster had an inner bellows so there was no seal at the MC.

After installing the new Metelli brake master cylinder and bleeding the system not once but twice, I still have NO decent braking!

To recap, I first bench bled the MC before installing it. Then, after bleeding the system, I drove the car to try the braking and it was very weak. So, thinking there might be still air in the system, I bled it a second time, this time making sure it was done very thoroughly. But I still have no decent braking power.

There is definitely an improvement when comparing to the old MC. With the old one, the car would not hold still (with the brake pedal pressed down) on even a slight slope, I'd have to press down the brake pedal continuously. With the new one, pressing the brake pedal a couple of times and then holding it there keeps the car permanently still on the same slight slope. But as I said, braking is not sufficiently strong to drive the car around safely.

Bill, I'm not sure whether the faulty MC had leaked fluid into the vacuum booster, but I'm inclined to doubt that since the fluid level in the reservoirs did not go down when the older MC was in the car.
However, after installing the new one, the engine no longer shuts off when the ignition if turned off and the key removed. So, this seems consistent with a malfunction in the vacuum system. The new O-ring at the seal was identical in size to the older one. But perhaps I didn't clean the surfaces sufficiently well or something.

Should I take off the MC once again (what a PITA, it was difficult to reconnect a couple of the hard lines) and take a look at the seal? Any thoughts on this?

first, I'd verify the brown line from the IP to the vacuum switch is intact on both ends, make sure you did not bump off a vacuum line.
if the pedal is not hard you don't have to worry about the vacuum seal on the booster.
as for braking problems, if the new MC is not holding the car still... you could have stuck calipers... pull off each tire, and get a screwdriver in there, and verify the pads move back and fourth.
what method are you using to bleed the brakes?

This might be your problem:

http://www.peachparts.com/shopforum/diesel-discussion/189944-worn-front-brake-pads=high-pedal-effort.html

I think anyone who is working on or planing to work on their braking system should build or invest in a pressure bleeder..

http://www.peachparts.com/shopforum/diesel-discussion/292387-diy-pressure-bleeder.html

Power Fluid Bleeder - Motive Products MOT0100 | FCP Euro

I'll check that brown line again tonight, but I remember checking all of the vacuum lines when the engine stopped shutting off via the ignition key (I had experienced a similar problem a few years ago and it was due, precisely, to a vacuum line coming off).
The brake pedal is not hard at all, it feels normal as it usually does with brakes that work well. I just have to press it two or three times to achieve some noticeable braking and then the braking is still weak.
The new MC DOES hold the car still on a slope as I indicated in my previous post.
To bleed the system, I used the power bleeder I built recently (works very well), you can see it here: http://www.peachparts.com/shopforum/diesel-discussion/341508-240d-brakes-how-do-i-bleed-master-cylinder-2.html