Long, soft brake pedal

[gjz30075]

I'm trying to get a good brake pedal on my S3 but having a hard time just getting any semblance of
a proper feeling brake. I've rebuilt the GT6 front calipers, rebuilt the rear calipers, new braided
soft lines, two master cylinders and three rebuild kits later, I'm nowhere I should be with brakes.

They will pump up a bit but the pedal stroke is still too long. So, it was suggested to me that maybe
my homemade hard line that runs from the master to the brake light junction is the culprit. First,
I had put a junction in the middle because it happened to be easier to assemble and maybe the junction
is sucking air. It is not leaking, however. No leaks anywhere. But the real culprit might be that
my 180 degree loop is too high, ie, higher than the resevoir, as pointed out in the pic. What would be
the consensus on this possible 'faux pax'? Any other ideas would be helpful, too.

Additional info: There's virtually no play with the pedal push rod. It almost immediately pushes the
master's piston.

[nmauduit]

if there is no leak it won't suck air (air is at 14.7 psi only, vs. in the range of 1000 when pressing hard the pedal)

if there is a high point in your setup an air bubble certainly can stay there : to move it you'll have to pull or push the fluid fast enough (viscosity and surface tension varies with the fluids). In some cases tiltling the car may help solve the issue (workable only to a certain extent obviously).

If the loop you show on the photo is likely the only culprit, you may want to explore "back purging" by pushing fresh fluid rapidly from a caliper. Once the bubble is removed there you should be able to purge the regular way afterwards, though a high point will tend to collect micro bubbles over time and risk becoming a nagging nuisance over time.

[alan.barker]

I suggest you pump the brake pedal then block the pedal down as much as possible. With a piece of wood between the pedal and the seat. Then undo the brake pipe connection a little where it leaves the Master Cylinder to let the air bubble out, tighten. Then remove the wedge on the Brake Pedal and test
Alan

[RichardHawkins]

Sometimes one can be too gentle when bleeding brakes, and any trapped air remains in place. You might find that if you depress the pedal quickly ( like in a panic stop) that air will be moved through the system.

A long time ago a Europa owner I knew had somehow got a wheel cylinder with a bleed nipple that was not at the top. I know it sounds really basic, but it took him ages to notice what his problem was.

Richard Hawkins

[crannyr]

Assuming all else has failed, the single best advice I have read on this subject is to wedge the brake pedal on with a length of wood overnight and leave it. In the morning all should be right with the system. As I understand it the constant long pressure forces mico air particles out of the fluid. Worst case do it 2 nights in a row. Good luck.

[gjz30075]

Ok, Alan and Cranny, sounds like a plan. I'll let the forum know how it works.

[paddy]

It wouldn't explain softness, but a long pedal can also come about because the piston on one or more calipers are withdrawing too far into the caliper when the pedal is released. This happened to me after a recent caliper rebuild. The telltale sign is when there's clear movement of the pad possible with no brake pressure - eg with a gap of 0.5-1mm between the pad and disc.

Paddy

[gjz30075]

It wouldn't explain softness, but a long pedal can also come about because the piston on one or more calipers are withdrawing too far into the caliper when the pedal is released. This happened to me after a recent caliper rebuild. The telltale sign is when there's clear movement of the pad possible with no brake pressure - eg with a gap of 0.5-1mm between the pad and disc.

Paddy

Interesting and a good answer. I do have free movement of three pads in the rear, ie, I can
wiggle them, without pedal pressure applied. My Sprint has one very slightly loose pad in the rear
and it has an excellent pedal. What's the fix?

[types26/36]

If you have normal flexible hose's (not sure you can do it with aro-quip hoses) clamp each hose at a time with a suitable clamp and try the brake pedal each time, it will isolate each caliper and will show if the problem is within the caliper.
https://www.ebay.co.uk/itm/Laser-4976-h ... ctupt=true

[gjz30075]

Brian, I've got Aeroquip lines, but you're on the right track for diagnostic purposes. I just need a way
to isolate them.

[paddy]

What's the fix?

What I did is to fit an old (thinner) pad and pump the brake pedal, so the piston moves out by a 4-5 mm, which unsticks the seal from the piston. Then gradually force the piston back in (eg with a g clamp) until the new pad only just squeezes in. The back and forth movement re-seats the seal on the piston and in the caliper groove in some way. That resolved it for me.

Trying the procedure the opposite way by pushing the piston all the way in didn't work for me, because with new pads the piston is virtually all the way in already, and there wasn't enough travel possible.

Don't try your luck and push the piston out without an old pad, or something at least, to limit the movement of the piston - if it comes out too far you're toast. Also, do one at a time so you don't run out of fluid in the master cylinder.

Paddy

[gjz30075]

Thanks, I'll try that. Sounds like two separate problems here. Right now, the pedal is forced down
hard and held in place with a board. I'll see what happens tomorrow.

[Mazzini]

GT6 calipers on a S3? So you have +2 calipers? Are you still running the original size MC for the smaller front calipers?

[gjz30075]

Yes and yes. I got the car this way and thought it a nice upgrade, rather than try to source the
smaller calipers and rotors (and probably spindles and uprights). Master cylinder is a .70

I know there are many posts on this upgrade but I've never paid that close attention to them because
the Sprint didn't need the upgrade. I don't remember reading about a change of the master to
accomodate this. Hmmm.....

[Mazzini]

I think this is your problem. I can't remember which size MC you need to go to, but I believe this is your problem, not air in the system.