Hiya folks. I'm doing my first brake work on my 81 300SD, and have replaced the following: all rotors, all calipers, all pads, all soft lines, all hard lines, master cylinder, master cylinder to booster seal. I haven't replaced the booster or any vacuum lines. I've bench bled the MC and bleed the lines a couple of times now. I'm not sure how the pedal is supposed to feel but this is what's happening.

With the car off I can pump up the brakes and they hold pressure. I turn the car on and lose some of the pressure. I can pump them back up, but it slowly fades, not all the way to the floor, and I don't feel like I'm losing braking power (that could be the newbie talking). I've only taken it up and down the street a dozen times (<30mph) but it feels like I have a good range of brake control, and they seem to be there when I need to stop fast. Also, the fading lessens the more I drive.

I read that fading is related to a bad MC or air in the MC or lines, but I've bled both a couple of times now (furthest first, with someone helping, bench bled, etc) and it still persists. But the brakes seem to be holding fine. There's no brake fluid leak into the booster with the old or replacement MC that I could find. I've also read that vacuum leaks can cause problems and the area around my vacuum pump is messier than I would like it to be. That being said, I haven't noticed any other vacuum related issues (run-on seems fine <1 sec, haven't driven long enough to really notice shift points, seemed a bit hard but again, I read that's how these cars are).

Any ideas? Defective MC? Should I go snag a vacuum gauge? Or are these brakes just softer feeling than newer cars? Thanks for reading.

~bri

The brake pedal should go down slightly when the vehicle is started (if you pumped the brake pedal and held it "tight" before you started the vehicle) this is a sign the Vacuum brake assist is working.

Once the vehicle is started, the brake pedal continues to go down?

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1982 240
460,000 miles total
140,000 on Rebuild

Ok, so the power assist sounds good, but yeah. Personally the brakes feel silky smooth and responsive (but not touchy), but it doesn't stay firm when the car is on. Like I said, it doesn't drop to the floor, but fades slightly if I try to hold braking pressure. It seemed to get better as I was driving, but it'll do this when parked. I'm going to try a longer test drive see if I can get a better feel for it.

~bri

It sounds like you still have air in the lines.

How "exactly" did you bleed the brakes?

If you are doing it the "old fashioned way" it is VERY easy to introduce air into the lines. Either by pumping a little too fast or allowing the reservoir to dip a little too low.

The best way is with a pressure bleeder. Or you can use a vacuum pump in a pinch.

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AJ

1985 300D (SOLD)

Yeah, I went the old fashioned way, reservoir never got low, but the bleeder screw threads would be my bet as for air getting in the lines. Or pumping too fast... my helper is kind of an excitable guy. I had a buddy pump up the brakes, hold down the pedal, then I would release the pressure, re-tighten and release the pedal, and repeat, a bunch of times for each caliper. I can try again with a pressure bleeder.

This thread mentions something about being able to hold the car at a stop sign (highlighted, 2nd post from bottom) http://www.peachparts.com/shopforum/showthread.php?t=219745&highlight=stop+sign

I don't quite understand the wording in their post, or what it's testing for, but I just tried this and as it pumps (3-6 times, depending on how hard/fast I pump) up I lose braking power and the car will start to roll, at which point I have to press a bit harder (maybe more how the brakes are supposed to feel?) so they will start to work again.

I'm bringing the car in for some new tires and an alignment in about 45 minutes, so I'll see if they can just bleed it for me while it's in there.

FWIW, before I stopped driving it, I had soft brake issues, but one day I had a sudden hard pedal/no brake problem (luckily I had just pulled into a parking lot), but just once. At some point in the past 7 months since, I noticed a vacuum line became disconnected off of the booster line. Couldn't tell ya if it made a difference, don't know how much it matters, but it basically sat since then.

I'll report after I get back from the shop.

~bri

Shop didn't have time, so I guess I'm off to buy a pressure bleeder. Noticed the brakes aren't fading as much as they were, if at all, but I only took it for a short drive. Still doesn't feel "firm" though.

~bri

About 1 1/2 Years ago I replaced my MC with a rebuilt one. It faded a little when installed it but at the time I needed to get the car on the road; but it never felt right (I did several bleeding jobs trying to cure it). After that 1 1/2 year the Pedal went to the floor. My past experience with good rebuilt ones is that they last atleast 4-5 Years. That MC died too soon.

I replaced it with a New not a rebuilt one and now have excellent braking.

In my case the MC was evidently poorly rebuilt to begin with.

If you installed a rebuilt one you could also be having the same problem.

The cause could be an internal leak due to pitting in the MC bore or if they oversized the bore too much to remove previous pitting causing some of the fluid to pass by the internal seals instead of going to the Calipers.

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84 300D, 82 Volvo 244Gl Diesel

I appreciate the heads up, at least that gives me some kind of time frame to anticipate failure.

I drove it around a bit yesterday and the fading seems to be fading, errr, going away. Don't quite know what that means, but I'm going to have it inspected today and we'll see if it passes. We're in the same boat though, I need to get this on the road. I've had it since November and haven't been able to use it, and I'm moving on the 1'st so it would be great to be able to haul stuff with it.

Ok, off to the inspection station... wish me luck!

~bri

I would keep the receipt someplace where you can find it later.

Even though I put a new Master Cylinder on after about a Month the receipt turned up and I exchanged the defective one for another Master Cylinder due to the warranty. I will keep it as a spare and hope on down the road it works.

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84 300D, 82 Volvo 244Gl Diesel

If you are still leary and it doesnt feel right, chances are it is not. if there is air in the lines the possible failure is not something you want to think about. I had air in the lines on my coupe and what happened was they completely failed in heavy traffic. I think the fluid was able to boil. then after it all cooled down I had good brakes again. if you drive in heavy fast stop and go traffic, I would get a good pressure bleeder and do it again to be safe. there are also a few threads here showing how to build one from a bug sprayer. I built mine from a $7 sprayer from home depot and it work fine.

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1982 300CD Turbo (Otis, "ups & downs") parts for sale
2003 TJ with Hemi (to go anywhere, quickly) sold
2001 Excursion Powerstroke (to go dependably)
1970 Mustang 428SCJ (to go fast)
1962 Corvette LS1 (to go in style)
2001 Schwinn Grape Krate 10spd (if all else fails)

The brakes feel much better and it did pass inspection but don't think I'm taking this lightly. The fact that the symptoms fixed themselves is enough for me to be concerned. I'll get ahold of a pressure bleeder somehow and try to fix this in the next couple of weeks. Right now, my commute is 35 minutes half highway half suburb driving, but that'll change to about a 10 minute commute in a couple of weeks because I'm moving, so I'm going to be busy with that. So I probably won't get to it right away, but definitely in the next 2-3 weeks.

Receipt is definitely in a safe place. Once I get refunds for calipers and cores, I'm going to sit down and tally up the repair costs.

Thanks for all of the advice guys. After my ALDA banjo bolt cleaning this morning, an Italian tune-up, a fresh inspection sticker, and a good wash and wax job, she's really starting to come together.

~bri

My vote is for the MC as well, as long as there are no fluid leaks under the wheels anywhere. Sounds like you've kept a good head and kept the receipts for everything. I would just return the MC to where you got it and trade it in for either another rebuilt unit, credit toward a new one, or get your money back. They should be okay with it, as long as they care enough about their customers...especially after calipers pads rotors and hoses!

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth