Air in the brake lines?

[Christien]

Ehhh, another thread on brake bleeding... Sorry guys.

I replaced all my soft brake lines and refilled the system with ATE superblue using the Motive pressure bleeder (with fluid in the power bleeder, not the "just air" method). I've got stopping power, but it could be better. I doubt I could lock the wheels up. The fluid's all brand new and I haven't driven the car more than 200km since the change (all regular driving, no autox or track) so the fluid doesn't need changing. But I do have air in the lines. (MC should be fine - it's brand new as of last summer.)

So simple question - what's the best method for getting the air out? Let's assume my wife doesn't feel like pumping brakes for more than a couple minutes Just air in the power bleeder, watching the MC reservoir level? Tapping the calipers as they bleed? (I did that last time) Some kind of voodoo shaman chant?

Thanks.

[911pcars]

"So simple question - what's the best method for getting the air out?"

Nothing against your spouse, but under your direction, the bleeding operation may have damaged the master cylinder seals. If pedal went to the floor (assuming a used master cylinder), the additional stroke distance may have torn a piston seal. No amount of bleeding will fix this.

If so, your symptoms will be:
- Brake pedal goes slowly downward.
- Brakes can be pumped up to normal height, but with constant pressure, will slowly go downward.
- No outward trace of fluid leak.
- At least 2 quarts of fluid have gone through the system during bleeding with no improvement.

Repair: New master cylinder and more brake fluid.

Sherwood

[Por_sha911]

Sherwood: How much downward movement of the brake pedal is an indicator of a bad M/C?

[911pcars]

"How much downward movement of the brake pedal is an indicator of a bad M/C?"

Well, if the pedal slowly goes to the floor with constant foot pressure, that would be a sign. You should have a nice firm, consistent, solid pedal stopping about midway through the pedal range, if not before.

Sherwood

[Christien]

Sherwood, thanks for the reply. I was very careful to make sure my wife didn't push the pedal to the floor and break the seals - that's why the MC was replaced last summer, it was leaky. The pedal doesn't build up pressure with several pumps, which leads me to feel confident that there's no leak. I didn't use anywhere near 2 quarts of fluid with the last change (a quart is close to a litre, right?) I used less than one container of superblue, which is 1 litre - probably about 750ml.

Is it possible she didn't press down hard enough? When I did the last bleed I relied mostly on the pressure bleeder, and just asked her to do one set of 'pump, pump, pump, hold" per caliper.

[47silver]

I wonder if you could fashion a limit out of some 2 x 4 lumber so that the pedal will not travel too far towards the floor. This should prevent over extension.

[911pcars]

I've posted the special brake pedal limit tool previously, but I'm not sure Pelican offers it yet.



So the pedal is spongy but doesn't go to the floor?

Attach a clear nylon tube onto the bleeder screw (about 2'). Point the tube upwards, then open the bleeder. The fluid will rise to the level of the master cylinder and stop - whereupon you can see if any air bubbles migrate out of the caliper. Stroke the pedal gently to push more fluid and air out of the caliper. If there's air in the system, you can see the bubbles escape. Repeat on the other calipers.

I only mentioned 2 quarts since that seems to be how much fluid many end up using with a pressure bleeder. Definitely a waste of good fluid.

Sherwood

[ecm911]

Not sure i understand.

Pedal does not pumo up. Does it go to the floor?


Did you change brake pads? Different type?
Get brake fluid on pad or rotor - if need to replace nit just clean,


Is the pedal hard or soft ??

Long shot and would be evident with a very very hard pedal with poor stopping power - check the vacumn boost hose connection.

Good luck
Earl

[Christien]

Sherwood, I'll definitely give that a try - thanks.

Earl, pressure doesn't increase with pumping but does not go all the way to the floor, which as far as I understand is consistent with air in the lines. The pedal isn't hard or soft, kind of medium. I've felt what a brake line leak feels like in our old Lexus - it would build up pressure with pumping. I felt what it feels like with it goes to the floor no pressure - when bleeding.

There's no vacuum boost on a 72 - those started with midyears or SCs, not sure which.

[creag44]

If you are looking for a way to bleed the brake lines, easiest way I have ever found. Put a catch pan underneath all the calipers, open up all the calipers bleed screws and let it sit there for awhile. The fluid will eventually force the air out of the lines, without having to pump the brakes or anything. Once the fluid is coming out of the bleed screws steadily and there is no signs of the fluid stopping or "bubbling" out. Then tighten up all the bleed screws and fill your resevoir back up. Like I said this is the easiest and one of the most effective ways I have found. Granted it takes longer to do it this way, but it does get the air out of the lines. I hope this helps you with your problem.

-Mat

[Christien]

Mat,

Thanks for the tip, but I've got speed bleeders, so I could let them sit open for all eternity, and nary a drip shall pour forth...

[creag44]

Oh I see. Well best of luck. I will see if I can come up with anything else. Kinda brain dead as of right now

[Green 912]

What did you do besides the added lines? Did you rebuild the calipers, move the pistons or change the pads? Pedal feel will increase with vigorous pumping of the pedal. Bad MC, pedal will stop in normal place or close and then slowly sink to floor if you keep pressing on the pedal.

If you moved the pistons back here is a little trick that might bring back some firmness in the pedal. The piston seals might be pulling the pistons back too much. There is a way to reduce this. Pull each pad one at a time and pump the brake pedal just enough to push the piston in some. Now test fit the pad and carefully move the piston back just enough to snugly slip the pad back in place. Do this to all 8 pads. The pad must slip with some friction between the piston and rotor. This takes out the over pull back you get from new seals and pistons that moves in to the pad rather than back to make room for the pad. This is required in my book if you rebuilt a caliper or push the pistons back too far when changing pads. This effect will clear up in 100 miles or so but why wait?

2 Qts? I routinely flush a brake system with one quart of fluid and almost always have some left in the Motive and a small amount still in the can for top off after flushing if needed.

[Christien]

I didn't do anything besides add the new lines. I put new pads in earlier this summer (Metal Masters). I don't get any pressure building with repeated pedal pumping, nor does it slowly continue to the floor if I keep pressing. Since the last bleed, I've driven maybe 150 or 200 miles. Since the pad change probably more like 1000.