more braking news, sorry, that was bad, (long) - The 2002 Series Message Board

I may be way off, or I may be right; who knows? I see you haven't gotten much help on this, so what do you have to lose? However, I have a little time to think about your situation this morning before getting back to work on my '02, so here's the explanation of my opinion that your master cylinder piston is simply not returning properly.

The distance the calipers relax from the rotor is almost negligible; the pads actually seem to stay in physical contact with the rotors, so I'm assuming a few things: that your problem is that the front brakes will not allow the front wheels turn, the system had been recently working satisfactoriy before the new calipers, that it had not been sitting inactive for even a few months, that the pedal returns to its proper relaxed position, that the calipers are high quality oem replacements if not original BMW, that the caliper pistons were completely lubricated and compressed before installation with fluid and bleeding, that the rotors are oem if not orignial BMW, and finally, that the new calipers are installed with the bleed screws at the top, not at the bottom. (Sorry, most of us have installed a set of calipers "patas arribas" at least once, so don't take that as an insult of your skills.)

Note: The one way valve in the vacuum assist hose only allows the negative intake manifold pressure to reduce the effort required to press the pedal. It would not actually have much to do with the release of the front calipers unless it in some way reduced the ability of the master cylinder piston to return, but it is part of the system, so must be considered as a minor contributor at the least.

It could be that the rods between the pedal and the master have been misadjusted to increase the mechanical pressure against the booster side. The push rod from the pedal through the booster should not have any loose play, but it has to allow the master piston to return fully in order to allow the now depressurized fluid to return to the reservoir. I don't see why that would have been misadjusted in the first place, but it is a possiblity. To check, have someone push the pedal in and pull it in and out at the point of contact and listen to hear any motion of the linkage tapping at the piston inside the booster. If you hear it tap, I'm thinking it is not the linkage, but that the piston is not returning to make contact with the pedal linkage rod. I wouldn't jump to make adjustments to the linkage just yet.

If everything else in the system checks out, I imagine that your master cylinder might need a rebuild or just a good cleaning. Here's why:

The front calipers naturally don't release very much from the rotor, and they don't have any springs to help do that; it is just that the hydraulic pressure from the master should have been reduced by the normal return of the piston in the cylinder. There is a spring inside the rear end of the master cylinder which returns the piston to its relaxed position. The pedal pushes the piston in, but the pedal has its own return spring to bring it back up. The pedal does not pull the piston back out of the master cylinder, nor does the piston actually push the pedal back up. So, just because the pedal returns, it doesn't mean the piston has returned, therefore the piston may be maintaining enough internal pressure to keep the calipers firm against the rotors.

Note that reason the rear shoes don't seem to be a problem is because they have rather strong return springs to pull them back to their relaxed position. Consider that the rear return springs might even be adding pressure to the front calipers through the "stuck" master cylinder piston, and you have front calipers which are unable to release even the very slight amount necessary. These rear return springs should be helping to push the master piston back. You could, for fun, press the pedal, pull the hand brake lever, release the pedal, and see if the front calipers release, and then release the hand brake lever and see if the fronts release. It sounds funny, but Tthis would hypothetically manipulate the in-line return pressure from the rear to see if that is exacerbating the condition. It probably will do nothing, but again, it is a characteristic of the system, even if unlikely as a cause.

You will still need to determine if the master cylinder piston itself is actually returning. The piston will not return to position if it had some corrosion inside or if the internal spring were not working properly. I suggest separating the master from the booster without removing the lines. Take care with the o-ring between master and booster. Then see if the piston is flush with the open end of its cylinder. There is a retaining "e ring" and a rod seal and a couple of wahsers in the open end of the cylinder to prevent the piston from exiting and from leaking, so the relaxed piston with all its end pieces should be in firm contact with the "e ring." If it is recessed in the cylinder, it is not returning. Perhaps tapping lightly on the concave end of the piston with a dull center punch or a pin or even a bolt, would be enough to free the piston to get it to return (not likely). Another way to get the piston to return would be to pressurize the system from the other end, such as by pressing the piston into the caliper to create the back pressure which may push the master piston to its relaxed position. You can do that without opening up any lines, just a c-clamp will do. Otherwise, if you don't mind doing the whole bleeding process again, you can use air to blow greater pressure into one of the lines at the far end of the master cylinder, just be sure that the system is all assembled, as you can blow a sticky piston out of its caliper, along with a mess of fluid.

If the system had been inactive for some time, it could just need a little exercise to loosen it up. So, getting the piston to relax may seem to solve the problem. Still, since brakes are so critically important, I would want to be sure I've eliminated the problem completely.

I've just had my cylinder apart because I've been swapping in the m42, and I want to be sure my original system is in its best condition before I consider any upgrades, which I will. I found the costs of oem replacement or rebuild kits were high, and all my pieces actually looked very good, and it had been working fine anyway. So, I just cleaned everything really well and reassembled. It is not a difficult job, if you have to do it. I do have a good contact who thought he could get a new master cylinder for a somewhat reasonable cost if I need.

However, seriously, watch out for the Chinese counterfeit parts. The Chinese are counterfeiting everything you can imagine, certainly including aftermarket auto parts, which are everywhere, and they are labelled to look exactly like original pieces. They might even work, but since they are likely made with inferior materials and tolerances, I wouldn't trust them. I tend to stick with original BMW parts when I can and where they are the most critical. Sorry for such a long post. Keep us advised. Good luck. Jim