Brake Losing Pressure (new thread for clarity)

[hamann318is]

Hey guys,

I've got an older thread running, but at this point it's mucked up and kind of difficult to understand what's actually going on.

Currently I've replaced: the master cylinder (2 times, currently on a new unit), soft lines, rebuild the wheel calipers, new pads. A shop installed the second master and confirmed that the rebuild on the wheel calipers was good.

The problem: I lose pressure fairly quickly. After the shop installed the new master, the pressure was good for about a week, and slowly it has lost pressure. Right now I can still brake somewhat normally, but I have to brake through some "mush".

Things I've noticed: When replacing things at the end of the system (pads/wheel cylinders/soft lines) the brakes behave normally for about 3 months. This has happened consistently, whenever I upgrade something on the high pressure side, the brakes work for much longer.

Replacing things at the beginning nets not much time before I lose pressure. After replacing the master, I lost brake pressure within a week, both times.

The only parts of the system that I have not changed are the hard lines, ABS and brake booster. I do not think it's the booster, the pedal never seems to have a too stiff quality. Only on the recent master cylinder was the pedal very stiff, but I was able to brake normally. I do not leak fluid, so I do not think the hard lines are to blame. Every bleed shows air, so I know that air is getting into the system, but I do not know how or why it randomly happens.

I know there is a pressure regulator, but I don't know how that could be involved. Could the ABS cause these kinds of problems?

I pretty much come to you all in desperation. I've been dealing with this problem for almost 2 years, and it's becoming frustrating running down every rabbit trail. Does anyone have any insight from anything that I've said? Any help would be greatly appreciated.

[jlevie]

If you get air from the calipers each time you bleed the brakes:

1) Your bleed procedure is flawed
2) Air is being sucked into the brake system.

Pedal bleeding will work, but unless done properly there's a chance of introducing air into the system. To pedal bleed properly requires two people and close coordination with respect to pressure on the brake pedal and opening of the bleeder. A pressure bleeder makes this a one person job with no risk of introduction of air.

If air isn't being introduced when bleeding the brakes, it has to be being introduced during use of the vehicle. That could be at the master cylinder or at the reservoir to master connections (i.e., on the low pressure side). A leak elsewhere in the brake system would result in a loss of fluid.

Are the replacement master cylinder you've used new units or rebuilts? A rebuilt can be an iffy prospect. Although considerably more expensive, I only use new master cylinders now.

[hamann318is]

If you get air from the calipers each time you bleed the brakes:

1) Your bleed procedure is flawed
2) Air is being sucked into the brake system.

Pedal bleeding will work, but unless done properly there's a chance of introducing air into the system. To pedal bleed properly requires two people and close coordination with respect to pressure on the brake pedal and opening of the bleeder. A pressure bleeder makes this a one person job with no risk of introduction of air.

If air isn't being introduced when bleeding the brakes, it has to be being introduced during use of the vehicle. That could be at the master cylinder or at the reservoir to master connections (i.e., on the low pressure side). A leak elsewhere in the brake system would result in a loss of fluid.

Are the replacement master cylinder you've used new units or rebuilts? A rebuilt can be an iffy prospect. Although considerably more expensive, I only use new master cylinders now.

The first unit was a rebuilt, but this unit currently is new. The last bleeds have only been performed at shops. When you say reservoir to master, you mean the physical plastic unit on the top of the cylinder? Anywhere else that air could be getting into the system? (ABS unit/pressure regulator?)

I ask only because each bleed has yielded a great pedal, and then only after using the car does the pedal become mushy (after a week or more). That's what makes me think that it's being introduced because of a faulty component of the brake system.

I also wonder if maybe currently this air could be just a result of a new master getting "worn in". The brake behavior on this cylinder seems different, the pressure loss happened, but I've been driving the car around right now, and the pressure doesn't seem to be dropping as much as it used to or at all even. Is that a common cause, or am I just hoping for a easier fix?

[jlevie]

Air would have to enter the brake system on the low pressure side, which means where the reservoir connects to the master cylinder or where the master cylinder rod enters the master cylinder. A leak elsewhere would be on the high pressure side and would result in a loss of fluid at the leak. I can't think of a way you could have leak on the high pressure side that only resulted in the ingestion of air.

A leak where the reservoir connects to the master is a bit of a stretch as that should also cause a loss of fluid. So that leaves the master cylinder itself as the most likely source of air in the brake system.