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Discussion Starter #1
I've had this condition for years, now let's see your opinion...
So, in drive the problem comes basically only on slippery surface; when you release the brake pedal, wheels are still locked for a while and you can't control the ride like you should.. My car has passed inspections several times with this cond, brake forces themselves are good.
What's new:
front - calipers, pads, brake hoses
rear - calipers, rotors, rearmost lines, rearmost hoses.

Preliminary tests reveal that if I grap the pedal with my hand and pull it back, so do the brakes.
Between the lines from main cylinder to rear and rearmost lines there are hoses. Could those be plugged and cause this problem? Something with brake booster / vacuum things? Thanks already.
 

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Sounds like you need a new master cylinder. That's what cured this condition for me. The pedal would come back, but the brakes would stay locked and even hold on a hill in neutral for about 2 seconds. This was after new front hoses had been installed.
 

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The lines at the master cylinder generally stay in very good condition as they are dry and warm in the engine compartment, unless the car sits for long periods between uses. By that I mean more than one season without being started or moved. Are we talking about steel lines or braided hoses or rubber hoses? Hoses could be collapsed internally and produce your symptoms.
 
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It's easy to disconnect the lines from the master cylinder. Make sure to use a tubing wrench, which looks like a box wrench with one of the 6 sides cut out to slide the wrench over the tubing. It will grip 5 sides of the hex nut instead of only 3 sides like a box wrench does. This will prevent rounding off the nut.

When replacing, a master cylinder MUST be bled of all air before bleeding the air at all 4 wheels.
 

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If you have the original master cylinder it will most likely be aluminium. You shouldn't have a problem loosening the brake line fittings if you use the proper size flare nut type of wrench. It may help to spray some penetrating oil on the fittings a day or so in advance to help loosen things up.
Most of the replacement master cylinders I've seen have been made out of cast iron. It may be possible to just get a rebuild kit for yours.
 

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Discussion Starter #7
Just replaced the master cylinder and did all the bleeding, naturally from all the calipers as well. All went smoothly, thanks for your advices. Result: NO CHANGE whatsoever. Bit frustrated as I've replaced almost all the brake components and still a word describing overall braking performance of this ultimate 4-wheel vented disc setup is: poor.
Yeah passed another inspection with the condition and braking forces are "just fine". Brake feel is lousy, spongeous. I can't lock up the wheels on asphalt no matter how hard I press. Just tested so that removed vacuum from brake booster; braking performance is very very poor (as expected) but the pedal seems to return normally. Without booster the pedal feels much harder, as IMO it should be with booster as well.
Ideas? I'd quess something with the brake booster or some related valve, or something else... (I'm saving the original aluminium master cylinder as it probably works perfectly well).
As a note I just drove again my mother's 1989 5-series BMW with 390 tkm. Those brakes feel about 10 times better and reliable compared to my LeBaron, a good reference. Those are small 4-wheel solid discs.
 

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Spongeous-- I like that word!
I'm thinking that there's something wrong with the booster if it's returning with no vac on it. There has to be some sort of valving assembly somewhere on the booster. The booster has vac on the "wrong" side (i.e. the side towards which the actuating rod moves) from a normal pneumatic system. It uses atmospheric pressure acting on the other side of the diaphragm. The brake booster is directly attatched to the master cylinder rod and the pedal, so it moves with the pedal. If the vacuum isn't releasing, you'll be getting boost when you shouldn't and the pedal will feel soft. This could also result in it not returning. Similar symptoms could be present if the booster isn't releasing the air that comes in to fill the void made by the diaphragm.
You've already answered a very big question by trying the brakes with no vac on the booster. Since the pedal returns when the brakes are straight hydraulic, we know that there isn't a blockage in the brake lines preventing the fluid from returning. We know that the spongeous (new favorite word) feeling isn't from air in the system, because it goes away without the booster. So we've narrowed it down to the brake booster and related vacuum harness. Try getting a vac gauge on the booster line just to make double sure everything is good, and see if you can find the air intake for the booster. It should be inside the passenger compartment somewhere, immediately where the pedal rod goes in to the booster, since all booster designs I've seen draw air from that location. Make sure this is clean. Not sure if the booster is serviceable or not. I imagine no, but then again, the alternator "is" and you can do brushes on those.
 

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Discussion Starter #9
While bleeding the brakes today, noticed something which is probably related to this. When I press the pedal to the metal (with lots of air in the system, engine off), there is some jamming in the pedal return at some point before it is all returned. This can't be from the master cylinder spring as the master cylinder was replaced. I think there is a spring in the booster as well. Did not check yet inside the car behind the pedal if there is something... Could it be anything else anymore than the booster? And btw the booster valve-thing where the vac hoses go in the eng. compt. is new as well. All the flex hoses have been now replaced as well.
 
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