Air in Brake Lines or Master Cylinder

[Interlocker]

I almost feel like this question is too basic to answer, and I think I know the answer, but I'd just like to be sure.

New pads are set to arrive today, and I'll install tomorrow morning, but I'd like to know if I should re-bleed the fluid as well.  

The fluid was changed last week by my mechanic, who's quite good, but he may have had one of his helpers do the fluid (I know, I know... I *should* be doing all the work myself, but the car ended up there on a flat bed due to cooling issues, so I had some other things done while it was there to save time while on a real lift... My wallet hurts big time because of it, though).  

Anyway, before being dropped off, the brakes were less than impressive, but worked fine.  Now, however, they honestly scare me... and I don't frighten easily.     The pedal is long and soft, but if I plan the braking out with enough time, I can pump the pedal once and then attempt braking again and it's a bit better.  Not great, but better.

I'm assuming there's air in the lines, but I'm not actually certain of what the symptoms of a master cylinder failing are.  I suppose it could simply be that the rotors are glazed from the track outing and the pad material is just a bit too low, but glazing seems unlikely to me based on the limited track time it had, as well as the hard braking I've attempted since then to see if the issue clears up.  It hasn't.

Oh, and if I don't pump the brakes once first, but still attempt *hard* braking (threshold with near-ABS engagement) the car will pull to the left a bit first.  If I pump first, then attempt threshold braking, there's no pull in either direction.  :confused:

Anyone care to give their diagnosis?

[bmwman91]

Sounds like air in the system.  If the shop used a power bleeder, I almost guarantee there is air in there.  If not used exactly right they will pump air in, and even used perfectly it is still not as good as a by-hand-job (lol).  I had to deal with this sort of issue with my first car (I never did get all of the air out either on that one).  If there is air in the lines, you can likely get it out.  In the master cylinder, you might be able to bleed it out, but it could still get trapped somewhere in the ABS unit (that seemed to be my problem).

So, what to do:
First, get a helper or 2.  Bleed in this order:
Rear Passenger Side
Rear Driver Side
Front Passenger Side
Front Driver Side

Take it out & test.  After making sure the brakes work at a safe level, find an empty space somewhere and hit the brakes HARD so as to engage the ABS unit from ~60mph to 0.  Do this 2 or 3 times (assuming the brakes are good enough to engage it).

Repeat the bleeding procedure above if you were able to get the ABS unit to engage.  If not, then there is likely air in the master cylinder and/or ABS unit.  It is not something you can really fix yourself.  You will likely just make it worse fiddling with the MC and ABS unit.  Go to a dealer or BMW certified mechanic and have them use their special bleeding apparatus (it bleeds all 4 wheels at the same time and maybe engages the ABS unit, I do not remember exactly, and it is the ONLY machine-bleeder I would trust).  Better safe & a little more broke than sorry.

I tried various methods for getting air out of the MC (removing it & submerging it in fluid while pumping it, doing it on-car with the outlet lines, blah blah).  Nothing really worked well, so should I have this problem on my new car I will probably have to pay someone.

[D. Clay]

My experience with master cylinder failure is that it is gradual. It also will leak down while sitting at a red light. After you stop by pumping the pedal and hold the car with pressure on the pedal, the pedal will slowly go to the floor as fluid leaks around the worn cups.

[Deutschbag]

My girlfriend's GTI had a similar problem, the brakes were just really mushy and could not be bled, air would continue to enter the system no matter what. We replaced the master cylinder and it works perfectly now.

[M42boy]

Yeah, replace the master cylinder if you think it needs it.  But, like the godfather says...  start at the brake caliper furthest from the master and go around to the one closest.  

I still have some residual air in my lines as well.  I'm going to try to make a pressure bleeder out of those ACE hardware parts as the writup in the DIY section suggests.  I report on the results.  

You can buy this for $45, and it works really well.  Used a friends many times.

[Interlocker]

Well, I think I figured out the source of the squishy brakes.  As we were bleeding them, the hose coming out of the side of the reservoir *ruptured!* Brake fluid all over the freakin' place!!  :eek:

Turns out the line had a crack about 1/4" long, so we cut the line, reattached and ziptied it.  Funny that there wasn't a fluid leak there, but I think it was having a vacuum effect and possibly sucking a bit of air in.

Anyway, fixing the line, bleeding the system and replacing the pads made the brakes feel MUCH better.  I'm not longer frightened of the car... except for all the squealy noises coming form the AC pulley now, but I'll make a seperate post about that when I've got more engergy... Long day!

Thanks for all the feedback.