cjoenck

Do brake lines get clogged? I was changing my brake fluid as part of standard maintenance and noticed that the fluid on the rear right (i.e. furthest away from the reservoir) was flowing significantly less than from any other caliper when bleeding the lines. I change the flexible lines a few years ago... Coming to think of it, I am also hearing a grinding / rubbing noise from the rear right wheel that is speed dependent. Could the brake pads retract less if the brake fluid flow is obstructed to that caliper? Before I break open the lines, I am wondering if someone else here has had a similar observation. In other words, is it normal that you get different flow rates from the different calipers when bleeding the brake lines?

Goughary

It's more likely your bleeder is clogged.

And as to the pads in the caliper, look up plate lift on rennlist .calipers can get sticky and if the plates squeeze down on the pad, they won't retract.

cjoenck

All 4 calipers were rebuilt by Goldline Brakes in Seattle a couple of years ago, that why I didn't think of plate lift. Good idea though, I'll have to check that out... I am using a Motive bleeder, i.e. pressurize the reservoir, so if the bleeder is clogged, wouldn't that impact all calipers equally? If the bleeder is clogged, I would get less pressure in the reservoir than indicated on the pressure gauge of the bleeder and fluid would just dribble out of the calipers. The "funny" part is that only the caliper furthest away from the reservoir appears to have less less pressure.

Goughary

No no- the bleed nipple on the caliper. That gets clogged. And is likely the reason why it's running slow there.

A914MAN

If your brake fluid is very old, it's always a good idea to pressure bleed the system before retracting the pistons. Pushing the pistons into the calipers can push old contaminated brake fluid into the ABS system, and may cause problems. Pressure bleeding the system with fresh clean brake fluid insures you don't contaminate the system.

Porsche has always recommended a 24 month service interval on brake fluid. I wonder how many of us who don't have race cars do that?

cjoenck

Duh, of course. Now I feel really stupid

Goughary

You have no idea how many times i feel stupid.

It's all good. Thank god i feel ok breaking things. It's actually one of the rules in my garage...

Rule 1. No anger
Rule 2. If anything breaks while it's in my garage....your car broke itself.

JasonAndreas

DOT-4 fluid will congeal, whether age related or a result of moisture/contamination I am not really sure. When that happens and depending on where the problem happens, you will likely have to use the brake pedal to generate enough pressure to push it out of the caliper bleed valves. The Motive doesn't generate enough force. And sometimes when its real bad, you have to flush the calipers and the lines and the hydraulic pump with rubbing alcohol to completely clear them. The brake fluid will come out as a white goopy gel.

Goughary

Jason, I've heard that dot 3 will dissolve the crusty dot 4 but have never tried it

Do you have any thoughts?

Marine Blue

Another thought, it takes more pressure to move a fluid longer distances so if you’re running the same pressure to bleed each of the corners you would expect the furthest to take slightly longer and discharge at a lower rate.

JasonAndreas

DOT-3 is just DOT-4 with a lower boiling point. So I don't imagine it would work but I haven't tried either. Where-as I know rubbing alcohol works.

Goughary

But can you safely run alcohol through the entire system? Or would you just do that locally where you have a problem...

I'm interested...would be good to have a process for flushing out a c2 or a C4

JasonAndreas

I know a few cases of it having been pushed through the entire system but it was used as a last resort with the alternative being replace everything. On a C2 I would be concerned about getting it completely out of the brake master cylinder? I have only used it on calipers prior to rebuilding. If your pedal is hard and your pads are wearing evenly, or the issue is the spring plates, I would just stick with 2 liters of DOT4 to flush the brake & clutch lines. Can't imagine introducing a fluid as preventative maintenance that has the potential to contaminate the entire system.

A914MAN

Denatured alcohol, definitely NOT isopropyl. Using rubbing alcohol to flush a brake system is bad advice. All I've ever used though is good clean, fresh brake fluid.

cjoenck

I am curious about this last suggestion. I am not a mechanical engineer but I thought hydraulic fluid was essentially non compressible, hence the ability to transfer force lossless in hydraulic lines. If pressure at the farthest caliper is indeed lower than at the nearest, wouldn't you get uneven braking? Hydraulic pressure translates into brake pad pressure to the disks. If what you are suggesting is true, how does Porsche compensate for less braking in the right rear and most braking in the front left. I know there is a front/rear bias valve but I was not aware of an individual caliper based adjustment. I am now wondering if there is a difference between the left and right calipers to compensate.