w123 brakes misbehaving

[dakota]

The brakes on my 1979 240D have developed a problem. It's an intermittent problem as I can't make it happen every time. When it does happen it will happen like this. When applying the brake, the car will slow down like normal, but just before comming to a stop, the "feel" of the pedal gets real hard and the braking power seems to disappear. doesn't happen every time though...maybe 1 time out of 4 or 5. Also, at a stop, when pressing the brake pedal hard, it seems as if the pedal will very slowly, almost imperceptably, go toward the floor.

Is this the signs of a leaky caliper piston, or master cylinder? Or something else?

Thanks in advance for any insight.

[engatwork]

and make sure you do not see any fluids coming out of them but it sounds like a classic example of a master cylinder going bad. Easy job and the parts are not too expensive from Partsshop.com. You need to make sure you "bench bleed" the master cylinder before putting it on the car.
Jim
'85 300D
'95 E320
'97 CRV

[chowpit]

Follow Jim's advice and check the brake sys thoroughly. You may be losing assist also.

[be459]

For the first problem, I think your vacuum is low. The low vacuum cannot power the brake booster for too long. If you step on the brake pedal too many times (using up the vacuum power) before the final stop, it will happen and you possibly may not be able to shut off the engine at this time. To make the matter worse, braking reduces the engine rpm causing weaker vacuum to the brake booster. You have to find out if your vacuum pump is bad or you have a vacuum leak.

For the second problem, a sinking brake pedal is a sign of bad master cylinder.

David

[nellotare]

had similar problem on mine.. once it was the vaccuum boost and the next time it turned out to be the calipers.. right front was/had been leaking terribly.. calipers are not expensive to replace.

A way to check the vacuum boost is to turn the key once to the right, pump your brake pedal 3 or 4 times.. holding down on the last pump, then crank the car. If the brake pedal goes down toward the floorboard after car is cranked, then it is NOT the vaccuum boost. If it does not go down any further.. well, then have a reputable guy look at it. Good Luck!

[dakota]

Thanks you all for your replies!

From your responses, it looks like I should do the following things in this order:

1. Check for leaks at each caliper (and anywhere in between).
I suspect that this will not be the problem, as the fluid level appears ok in the reservoir, and I detected no leakage a few months back when I changed the pads. But I will do a thorough check and replace/repair any visible leaking component.

2. Check for vacuum leak or bad vacuum pump.
I'm a little foggy on this and what this involves. If I use Nello's test (pump pedal, and crank engine) and the test is negative, I'm assuming that it is NOT the vacuum pump, but could it still be the brake boost (diaphragm) that might be bad? There seems to be several different brake boost/diaphragm kits available.

3. Replace master cylinder.
If Nello's test above is negative, should I jump to this step?

I probably am looking at this with wishful-thinking as the third step seems to be the least expensive, judging from the prices I can find here in the Fastlane. I know that I shouldn't just replace the master cylinder, if I really have a vacuum problem.

Thanks again for all your help.

[nellotare]

I'm out of town.. visiting and on someone else's computer so I'm writing very quickly. I don't have my Haynes book with me, but the test I mentioned is for the Vaccuum Brake Booster.. not the vaccuum pump as I may have led you to believe. In any event if you could get your hands on a Haynes Manual then it has some steps to follow. Keep us posted.

[Mike Michalek]

It sounds like the seals in the master cylinder are failing.

[engatwork]

master cylinder are two different problems. I would replace the master cylinder first - everytime I have seen a brake pedal go to the floor after stopping and keeping my foot on the brake it was the master cylinder. Make sure you "bench bleed" it before mounting it.
Jim