w115 brakes advice

[Abbos]

Hi everyone,

I recently purchased a 230/4 W115 (1975).

I am working on fixing a few problems at the moment but the one I am focussing on now are the brakes.

When I press down on the brake pedal, the brakes respond but the pedal does not stay hard and it slowly disappears until it touches the floor.

Although the car does stop I'm not satisfied that they should function like this. Can anyone give me a few pointers as to the best place to start fault finding the issue?

Many Thanks,
Abbos

[Mike D]

If you are not losing brake fluid from a line or caliper then it is master cylinder time.

[Abbos]

Thanks Mike, that must be it then as I can see no leaks!

Is there a HOWTO on this anywhere or a basic set of DIY instructions/parts needed?

[Mike D]

Master cylinder R&R is an easy task. If you have never done it I suggest you have it done by any competent repair shop. It's not hard but the potential for an accident due to a minor mistake is there.

The brake system should always be your number one safety concern. There are no short-cuts.

It only takes about an hour so the shop shouldn't charge too much.

[Abbos]

Thanks Mike.

Would you be able to tell we what parts are needed so at least I can pre-order them and supply them to the garage as they normally put a large amount on top for profit!

I can also easily source used parts if that helps at all but I'll admit I have no idea what is needed so hopefully I can be pointed in the right direction.

[t walgamuth]

just a new or rebuilt mc.

and if you have any skill at all it is a very easy job.

just undo the lines, undo the bolts holding the mc and remove. replace and teattach the two bolts and the brake lines. try the brakes. if they feel spongy you may need to bleed them. often you can install a mc without bleeding.

there maybe a wire attachment for a brake fluid level sensor but i think your car is probably too old for that.

in my experience.

tom w

[Mike D]

Tom covered what you need. A new or rebuilt master cylinder, a large bottle of brake fluid, the proper size "flare" or line wrench, a wrench or socket set to fit the mounting nuts, a medium flat shank screwdriver and possibly a larger flat screwdriver or prybar.

Against my better judgement I will tell you how to do it. If at anytime you do not understand these instructions or feel uncomfortable about any of the steps, PLEASE do not attempt it! I have worked as a professional mechanic since 1973 and I can tell you stories about "do-it-yourself" brake jobs which would make you afraid to drive on the roads.

First, compare your replacement cylinder with the old.

The only glitch might be the master cylinder you purchase will not have the plastic resevoir and you need to pull it from your old one. This all depends on the supplier. Some have it and some don't.

If the replacement does not have the jug, place a small tray beneath the lines where they connect to the master cylinder. Remove the brake lines on the side of the master cylinder with the line wrench. Drape a shop towel or rag over the master cylinder/line connections.

Use the medium screwdriver to remove the electrical connections to the screw on lids. Lay them or it aside.

Without the engine running, slowly depress the brake pedal and release the fluid until it is below the plastic jug. You will need to press 3 or 4 times.

The plastic jug just sits in two rubber grommets and will come out with a little judicious wiggling and prying. If you have room, do this while it is still mounted. If not then you can mount the removed master cylinder in a vise and pull the jug. Drain and clean the jug. I prefer to use hot soapy water and blow-dry it with an air compressor but a rag and sunlight works just as well. Lube the nipples with fresh brake fluid or a LITTLE bit of petroleum jelly or silicone lube and slip into the grommets of the new cylinder.

Remove the rag and tray. Wipe dry ALL the brake fluid which you spilled on the fender well. Give it a generous spray of degreaser, 409 or whatever to neutralize the brake fluid.

Remove the two nuts on the back flange of the master cylinder where it connects to the vacuum booster (the big drum looking thingie). Wiggle and tug, the cylinder will slide forward and lift it out. Clean the receptacle where the master cylinder sits in the booster with degreaser or a silicone spray and a rag. Give those mounting studs a good wiping and perhaps a little dab of "never-seize".

Fill the new cylinder with fresh brake fluid. "Bench bleed" the cylinder. Slip the cylinder onto the mounting studs, replace the nuts and tighten. Replace the brake lines loosely. "Snug" the lines. Replace the drip tray. Loosen each line 1/2 to 3/4 turn. Have an assistant SLOWLY depress the brake pedal to the floor and hold. Tighten the lines, have your helper let up on the pedal, count to 10, loosen the lines and have your helper repeat the depress and hold. Do this until you are getting ONLY fresh fluid NOT "bubbly air mixed" fluid (usually only takes once or twice).

Tighten the lines securely, replace the electrical lines remove tray, clean spilled fluid and then "gravity bleed" the entire system.

Gravity bleeding is easy. Top off the cylinder, open the left front caliper bleeder, sit down, have a cold one, keep the fluid topped and wait until you have fresh clean fluid coming from the bleeder. Close the bleeder and repeat on the right front, left rear and right rear. Reverse order on a RHD vehicle (it has to do with doing the shortest lines first).

Done.

You should flush the entire brake system since it may have lots of little rubber particles from the old master cylinder.

Did I forget anything guys?

[t walgamuth]

i don't think so. excellent thorough description of the entire job. i didnt think about flushing the system, but unless you know it has been done very recently an excellent idea.

tom w

[tram]

BEFORE you invest in a M/C, please make sure that your pads aren't worn and bottoming out on an anti- rattle clip! I've seen this more than once- mimicks a bad master. Check ALL FOUR wheels.

[Abbos]

Mike, thanks very much for you post, very detailed in deed. The most I have done before is changing disc's + pads and bleeding a system.

I'll call over a friend who is a mechanic and although he doesn't know old cars at all i'm sure with the instructions you kindly provided we can probably get it done.

Tram - Thanks for the advice. I will certainly check this to see if it could be the cause.

Thanks again everyone for your help.

[Abbos]

I also read on another forum to keep the brake pedal pressed down and then start the car. If the pedal then sinks to the floor then it is probably the master cylinder otherwise if it doesn't then it could be the brake booster.

I tried this and the later occured. Should I still proceed with the checks that you suggested Tram or does this conclude that it is actually the booster?

[Tomguy]

There is an easy way to check the brake booster. Mine will hold a vacuum for days. Start the car up, let it idle for 10 seconds and shut it off. This is MORE than enough time to let the booster build up a vacuum. Wait an hour, then loosen the vacuum line to the master cylinder. If it doesnt leak air, you have a leak in it. If it loses vacuum after only 5 minutes, it's a substantial leak. Both of these are assuming the check valve is working correctly, as is the source of your "pedal test".

[techman707]

Originally Posted by Mike D

Tighten the lines securely, replace the electrical lines remove tray, clean spilled fluid and then "gravity bleed" the entire system.

Gravity bleeding is easy. Top off the cylinder, open the left front caliper bleeder, sit down, have a cold one, keep the fluid topped and wait until you have fresh clean fluid coming from the bleeder. Close the bleeder and repeat on the right front, left rear and right rear. Reverse order on a RHD vehicle (it has to do with doing the shortest lines first).

Done.

You should flush the entire brake system since it may have lots of little rubber particles from the old master cylinder.

Did I forget anything guys?

I've been bleeding brakes for over 40 years using the conventional method of having an assistant pump the pedal. I've never tried the gravity method and needing to bleed the brakes without any help, I thought I would try it. However, before I do, I just want to confirm something first.

When using the gravity method, you appear to state it is the REVERSE of the normal method, namely, starting with the caliper NEAREST to the MC instead of farthest, which is the normal method. Is that correct?

Thanks.

[Mike D]

Yup, it's the reverse of pressure bleeding. The reason is that generally you will have a large pocket of air which will create an "air lock" effect.The shortest line has the quickest (and shortest obviously) flow rate. The most air is always trapped in the shortest line. Once you break the "air lock" bubble it flows smoothly.

[wbain5280]

On a car that old, I would suggest a complete overhaul. IMHO.