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Car Forum / Mercedes-Benz Cars / April 2006Bleeding Bubbles: The Director's Cut
In the original version: '81 300TD Changed master cylinder No matter how much I bled the brakes, still got bubbles. Solution: bleed 'me the right way with two people: Problem solved. Meanwhile... The original reason I changed out the mc was that the pedal was grabbing too close to the floor. Since all the pads had over half their wear left, I figured it had to be the mc. New mc = same problem--The pedal is not getting to braking until it's almost all the way to the floor. Sure, it'll lock 'em up on dry pavement, but they don't feel the way my other MBs feel/felt. So, I'm gonna try all new pads and see if that does it. Any other suggestions for those who have traveled this road before me? Many thanks. As much as you dislike the low pedal - and I would too - give it a few days to firm up. Changing the pads at this point will only worsen the condition, IMHO. Too many variables spoil finding a cause - except if a "lucky number" is hit. That said, many years ago I once reversed the calipers left to right, right to left which put their bleeders on their bottoms. The air would still be in the calipers if my boss hadn't seen the mistake! > As much as you dislike the low pedal - and I would too - give it a few > days to firm up. Changing the pads at this point will only worsen the > condition, IMHO. Too many variables spoil finding a cause - except if a > "lucky number" is hit. What? Wait a couple of days? That's a new one on me. Also I disagree with the "new pads will make it worse" conclusion. New pads usually bring the pedal up considerably and might also change the position of the masters plunger, which can work around a problem there... > That said, many years ago I once reversed the calipers left to right, > right to left which put their bleeders on their bottoms. The air would > still be in the calipers if my boss hadn't seen the mistake! Good one. In a well designed system that would not be a possible error. Also, I agree with the previous posters who suggest using the old school pedal method of bleeding. Suction bleeders aren't as good. Marty Suggest you inspect the brake hoses on this old car as someone pumps the brake pedal, one may be flexing a bit too much. Didn't you have bubbles from ONE wheel? That's the place to start. I find it puzzling that most people here seem to think that new pads will bring the peddle up. I don;t see how this is possible. As the pads wear the pistons just keep moving out, with the space behind them filled with brake fluid from the reservoir. It should still take exactly the same peddle travel to generate the same force and movement of the pistons. So, it would seem to me that pads would have zero effect on how far the peddle goes before firming up. If it worked that way, we'd all notice the peddle going lower as the pads wear, but I sure never have seen this happen. You're right, providing you drive a modern car. Back in the days of drum brakes (1940s - mid 1950s) pre-automatic adjusters, cars were brought into the shop for a "brake adjustment" which involved expanding the star wheel adjustment to spread the two brake shoes exactly, as you say, to compensate for the linings' wear. All history now. Can you 'pump' the pedal up?. i.e. does the bite point raise up the pedal (away from the floor) if you pump the pedal rapidly?. If you can pump it up, there is air in there!. Otherwise, as other posters have stated, the pads may 'bed in', but that should not really affect how far you press the pedal. Only greater pedal pressure will be the big difference. Could it be that the master cylinder piston is stuck part way down the cylinder, such that you only have a small amount of motion that is actually pushing the brake cylinder. I've seen this once before on a car that had the cylinder incorrectly assembled, such that the piston stuck part way down the tube. The pedal always returns to the top since it has a return spring, but whilst bleeding the brakes, since the piston did not return all the way, you only had a short actual master cylinder piston pushing travel. Hence the braking effort occurred right at the end of the travel. It was a tough one to figure out. It was fixed with a new set of rubbers for the cylinder, and the assembly instructions for installing them followed to the letter. Cheers... Rob. >In the original version: >'81 300TD [quoted text clipped - 13 lines] >suggestions for those who have traveled this road before me? >Many thanks. The brake bite at the same place no matter how much i pump them. i'm ordering all new brake hoses and will see if that helps it. I've had three different mc's on this thing and once the air was out, they all did exactly the same thing. I'm also swapping out a suspecious wheel cylinder. I'll post the results as they come in. Thanks all for your input. |
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