Loss of all brake pressure, losing no fluid, bleeding only temp. fix

[hamann318is]

I agree that my lines are probably horrid. I'm going to replace them for sure.

Is there anything I can check for in determining whether or not the MC is actually bad? It seems fine until I get it on the highway, where it just goes to hell.

Andrew was right. The lines were pretty old, and after replacing them and bleeding the system I had problem free braking for about 1/2 a year.

Recently, the problem has come back. I lost most of the system while just normally driving around town. Bleeding the system yesterday showed some interesting things:

- The master reservoir is still at full
- The rear passenger side line was FULL of air
- The remaining lines just had dirty brake fluid - no air

I've been driving around town for only a day, and I can just tell that I'm slowly losing pressure. Do these symptoms reveal anything new about the problem in the system?

[jlevie]

I would tend to expect a bad master cylinder to affect both front wheels or both back wheels, depending on which part of the master cylinder is bad. Finding air in only one rear line tends to implicate just that part of the circuit, perhaps a bad caliper seal? Usually one would expect a bad caliper seal to cause a leak, but maybe it only leaks in one direction...

Are you getting equal wear on both sides of the rears? If not, perhaps one side is blocked and air being pushed into the system at the master cylinder is only reaching that side.

Ordinarily I'd want to properly diagnose any problem so I'd know what failed and why. But brakes are just too important, so my approach in this case would be to rebuild or replace all of the calipers and replace the master cylinder with a new (not rebuilt) unit. That may not solve the problem, but it eliminates any of those parts as being the cause.

[SamE30e]

Sounds like a master cylinder seal.

[hamann318is]

I would tend to expect a bad master cylinder to affect both front wheels or both back wheels, depending on which part of the master cylinder is bad. Finding air in only one rear line tends to implicate just that part of the circuit, perhaps a bad caliper seal? Usually one would expect a bad caliper seal to cause a leak, but maybe it only leaks in one direction...

Are you getting equal wear on both sides of the rears? If not, perhaps one side is blocked and air being pushed into the system at the master cylinder is only reaching that side.

Ordinarily I'd want to properly diagnose any problem so I'd know what failed and why. But brakes are just too important, so my approach in this case would be to rebuild or replace all of the calipers and replace the master cylinder with a new (not rebuilt) unit. That may not solve the problem, but it eliminates any of those parts as being the cause.

That's the direction I'm going. I'm going to rebuild all of the wheel calipers based on the fact that that one had all of the air.

I'm going to take off the wheels tomorrow and take a look around, so I will let you know if I find that wear has been uneven.

What do you make of the 1/2 a year without any problems? I had replaced lines and pads, if there is a leak in the caliper, could the new pads have pushed the piston back too far for the leak to be seen until now? I'm just wondering why it was okay for a seemingly long period of time.

Thanks.

[jlevie]

I can't explain the half year with no problems... I strongly suggest a new master cylinder as a part of the fix as I feel it is implicated, somehow. You can get a new master cylinder (not a rebuilt) from Advance for just under $200.

[hoyabmw]

just because the problem was only in the right rear doesn't mean much--that's the first one you bleed, so most of the air will come out there. It sounds like the problem is in the rear. The rear is a single line to the back with a split in the center by the diff. Did you change the soft lines there? I also wonder about the proportioning valve directly under the master cylinder, if your car has it.

[jahnaboi]

Could always be the rear caliper piston is leaking... the reason why you wouldn't see is that when applying break pressure at higher speeds it would heat up the rotors/pads burning the leaking fluid.

[mintscorpion]

Did you bench bleed the MC before you installed it?

[hamann318is]

just because the problem was only in the right rear doesn't mean much--that's the first one you bleed, so most of the air will come out there. It sounds like the problem is in the rear. The rear is a single line to the back with a split in the center by the diff. Did you change the soft lines there? I also wonder about the proportioning valve directly under the master cylinder, if your car has it.

I don't think the car has a proportioning valve, what does it look like? We also changed all of the soft lines (2 front, 4 rear).

Could always be the rear caliper piston is leaking... the reason why you wouldn't see is that when applying break pressure at higher speeds it would heat up the rotors/pads burning the leaking fluid.

The only thing that makes me think no leak is that the reservoir is always at full.

Did you bench bleed the MC before you installed it?

We did not, but the only thing is that we've probably bled the system over 10 times since it's been installed, which makes me think that it probably has most of the air out. Especially when considering that it worked fine for 1/2 a year. I tend to think that the master is bad instead of not enough bleeding. Do you think that even with all that bleeding on the car, we could still have air in the master?

[jlevie]

There is a proportioning valve for the rear brakes. Follow the the line from the master cylinder towards the rear and you'll see it.

The fact that the brakes were fine for a while strongly suggests that you had the air out of the system for a while. Bench bleeding the master is always a good idea when replacing a master cylinder as it makes the flush and bleed process much faster and easier. But a through bleed will clear air from the system regardless of whether the master was bench bled. That's especially true if you use a pressure bleeder and push a measured 250cc through each rear caliper and 150cc through each front caliper.

[Z3Jonathan]

pump the brakes before you go in for the night and when you wake up see if the pedals soft if it is the boosters would be the suspect.

Usually a bad booster would cause the opposite, which is for the pedal to go hard.

Just concerning booster (let's ignore the M/C for sec), if you run all of the boost out (pump the pedal with the engine off), and let it sit over night, I don't think it's possible for the pedal to go soft. Atleast not from a bad booster. Generally, the only way the pedal will soften up is if the booster has vacuum, which even if the booster is leaking won't occurr with the car just sitting.

However, if you DON'T pump the pedal before turning off the engine, let the car sit over night, and the pedal has gone hard, that is sign the booster is starting to leak vacuum.

[hamann318is]

Hey guys,

New developments in the saga of my brake problems. I rebuilt all the wheel calipers, and magically the car was completely fine... for about 4 months again. I was driving yesterday, and up until the drive home, the car was totally normal. During the drive home, the car just started to lose pressure right after a quick acceleration (think to 7k in 1st), and now I have the same problems as before. Would that have anything to do with anything? I'm going to get the master replaced as I have no idea what the hell else could be making those problems.

Also, regarding the booster. I usually don't pump/use the brakes when turning the engine off, and my pedal is never hard to push in the morning. It's generally same old same old. That's why I don't think I have booster problems.

Anyone gain any new insights from all I've replaced (wheel rebuild, soft lines, pads, bled many times) and the fact that it will sometimes work fine for just a bit and then lose pressure?

[dmwe30]

sounds like the master cyl it could be getting air in from around the bushings they can be bad brand new i just had to order 3 masters for a customer till i got one that didnt leak it happens alot especially if its a reman i swear they are drunk when they put those together

[Liam]

Best guess, Master Cylinder.

[hamann318is]

I'm now on a new (not reman) master cylinder. The problem was fine for a few days, but now something interesting is happening. The best way to describe it is my brake pedal is inconsistent. Generally it feels "normal", but sometimes it's stiffer and others it's softer. When I'm on the highway, the pedal generally has a harder feel, but when it's around town sometimes it's kind of normal, and sometimes it's softer (like the cylinder is going bad). I can't really tell if the problem is coming back or not, but I have a feeling that it is.

I only say this is a problem because right when I got the car back, the pedal was very firm, nice feel. I'm wondering if maybe this is just air from the new cylinder finding its way out, but I doubt it as a shop installed it. They also checked my rebuild of the wheel calipers, all fine. What occurs is that air is getting into the system, but with a new master and fine wheel calipers and NO leaks, I have no idea at all as to why this is happening. Any ideas?

Also, could the brake pressure regulator be causing this problem?