F451

This may have been covered in this thread already, but how does the factory fill the brake system during manufacturing? It seems like they would be dealing with the same issue, essentially a dry system.

They must have specialized equipment, but I wonder if its just designed for quickness and other wise there is nothing special about what they use - in other words they fill it from the top, or they fill it from the bottom.

Anyone know how they do it?

dr bob

Nicole mentioned previously that the factory vacuum-fills the system. I'm not sure exactly what that means, especially after watching the video of the Phoenx vacuum bleeder with all the bubbles shown during their demonstration. I'm speculating that a hard vacuum on the system will get the air out, and that we will then be bleeding fluid in rather than bleeding air out.

F451

And I just put my brakes back on a few weeks ago after putting on new lines, painting the calipers, new rotors, pads, etc. The brakes had sat for at least a few months with the lines disconnected and the calipers off waiting for paint while I waiting for time to tackle this. Most if not all of the fluid drained out.

I've been following this thread and was worried about how things would turn out for me. I used a power bleeder and had pretty good results. My pedal action feels about 99% normal I would say. I begins to engage normally, but is not as firm as I would like for the first say 1/8 of travel. Then it firms up nicely.

My abs is not functioning (problems with the abs wiring harness) so I can easily lock up the front tires, so lots of braking force (I'll be looking into fixing the abs system soon).

I did two bleeds after putting everything back together, but before running it on the road. First did a quick one with the power bleeder to get the fluid in and to get out as much air as I could. Did the brake mc first, then started from the farthest rear bleeder and worked my way back.

Had a somewhat mushy pedal - not surprised there.

Did the second one the with the power bleeder. This time I started with the brake mc, then worked my way out from there - inner drivers side front, outer drivers side front, inner passenger side front, and so on.

Got pretty good results and feel safe with it on the road. Will be doing another bleed soon with some of the local 928 gurus to see what happens.

I'm also going to get one of those guys in my car to tell me what they think of the pedal pressure and action prior to bleeding for the 3rd time as I'm wondering how my brake action compares to theirs.

A part of me is wondering if my brake action is fine and I'm just being paranoid.

On the other hand, another bleeding with the local experts can't hurt.

Bill Ball

Dr. Bob: We talked about doing some kind of evacuation with a vacuum pump, but a few concerns could not be satisfied. Creating a vacuum would suck all the components in an unnatural direction, potentially inverting the master cylinder seals, for example.

F451

My 928 cents, just my humble opinion, usual disclaimers, etc.

No offense to the crew as you guys have forgotten more about 928's then I'll ever know I'm sure, but I have a hard time believing there is still air in the system after 18 bleeds with various methods .

And especially because you guys are so collectively knowledgeable, you're doing ubber bleeding that I'm sure would work just fine on an otherwise healthy 928 braking system.

If it was me, I'd start with what has changed just prior to the problem cropping up, I think as Stan has suggested more or less.

I'd replace the lines, bleed and see what happens.

Next, I'd leave the old lines on, and throw on some different calipers and see what happens.

Do you have the old lines? Assuming they were replaced because they were old, and not leaking, they should hold pressure just fine for the purposes of the test.

How about calipers? Can you borrow a couple of fronts to use for a test? Put on one, test. If no changes, put on the the other, test.

It seems like you guys have checked just about everything else.

I'd go with Occam's Razor. Start with what was changed just prior to the problem cropping up and go from there.

Good luck and I'm eagerly awaiting the good outcome, I know you guys will solve this and we'll have another good data point for the 928 faithful.

Ed

Bill Ball

I have no particular expertise with brakes, so maybe we should pull the calipers and try others. My problem is that I just don't see how the calipers could account for the behavior unless it is just still trapping air. Calipers are so simple -- piston, cylinder, seal. We are going to unbolt them and do some gymnastics to see if we can shake loose some trapped air in the calipers or the soft lines.

Ed Scherer

Forgive me if this has already been covered (there have been a few threads on this, some rather lengthy, and it's been awhile since I've read some of them), but if there is air at the caliper end(s) somewhere, where might that be and how would you dislodge those pesky bubbles?

I already mentioned (back in post #65 in this thread) that I had a bit of a battle with some air in my rear calipers after I removed them and painted them, and also in my front Big Reds that were brand new (and empty, of course) when I installed them.

That air was not entirely removed by pressure bleeding until I removed the calipers and rotated/tilted/shook etc. them to dislodge the last bit of trapped air.

Also, I hunted around a little and found somebody (not necessarily discussing calipers of the same construction, though) who recommended making sure that pistons are pushed back into the caliper when bleeding, theorizing that if the piston is far enough out, it can leave a spot in the bore that can trap air. I'm not sure what the internal construction of the calipers looks like (I'd like to see a cutaway diagram of these calipers), but that sounds plausible. You guys who have rebuilt your calipers know better than I whether or not this would hold true for the 928 calipers.

Also... (this is a stretch, but...) I don't know what the internal diameter of the brake hoses are, but I suppose if they're large enough and the bleed flow is slow enough, air could actually get trapped at a high point in the hoses. So perhaps it wouldn't hurt (especially if you remove the calipers anyway to rotate them, bang on them, etc., to dislodge air bubbles) to also elevate them such that the caliper end is clearly the high point) while bleeding to assure that any bubbles in the hoses have a clear shot to the caliper.

F451

Doing the caliper removal, elevating, rotating, rapping & tapping routine would certainly be worth a shot. Getting the lines to the point where they are angled up to the caliper seems like a good idea as well.

I do recall when I painted my calipers that I turned them every which way in an effort to get all of the fluid out once I had them off the car. It surprised me how much came out as I rotated them around to different positions. I was trying to make sure none of the fluid would leak out during the painting process and ruin my paint job.

When I reinstalled, I did some tapping of my calipers when I was bleeding when they were mounted on the car and I do recall getting some little bubbles out - I'm pretty sure they were from the calipers and not from leakage around the bleed screws, as the little bubbles appeared as I was tapping, and went away when I stopped. But who knows. It made me feel better as I felt I did everything I could do with the bleed.

Bill Ball

Right, we don't think we have tried elevating the calipers yet, although that should have been done right after they were rebuilt during the initial bleed. Anyway, Nicole and I already plan to do that.

dr bob

Bill-

I'm thinking that with the MC piston in the relaxed position, the ports from the bottom of the reservoir will be connected directly to the two circuits. The piston seal between the two sections will see the same pressure on both sides so no way to damage that. The rear seal, the one at the booster pushrod end, will have low pressure in front compared to atmospheric pressure at the rear, so the difference would tend to draw air in around the seal, forward into the reservoir. Hence my previously voiced concern. But that differential would be in the same direction as the seal is facing, so no danger of rolling or damaging the seal. Further, vacuum would tend to draw the reservoir tighter to the MC so grommet seals there shouldn't be a concern. Those are essentially bi-directional, and folks put more than 15lbs into the reservoir with their power bleeders. I don't thing that the vacuum will collapse the reservoir, casual guess after looking at the reservoir shape and wall thicknesses.

So my worry list includes the possibility of drawing air in through the rear piston seal. I need to figure out a good way to meter fluid back in through a bleeder port (possibly ALL bleeder ports) without allowing air in. Same problem the Miti-Vac bleeders at the calipr ports have except that air will be drawn into the car rather than drawn out through the hose to the Miri-Vac pump. No way to see if there are air leaks at the threads. Maybe an adapter fitting and a valve at each caliper? In the meanwhile I have my MC cap with the quick-disconnect fitting, and will fab a vacuum connection for that with a flare nut adapter that will take a common 1/4" AC hose connection from the vacuum pump. I think it would be a good idea to trap that line so there's less chance of aspirating brake fluid into the pump; tha would be a bit of a disaster. For basic testing, I have venturi-type vacuum 'pump' that runs on compressed air. Three stages, about 27" of vacuum on its little gauge. A puff of fluid through that would make a mess but not scatter pistons and vacuum pump oil all over the benchtop.

I'll be working on this some more, starting in a few weeks. We can talk about it more at OCIC. I can bring some of the pieces I have for a show-and-tell brainstorming session. If you have a spare/junk caliper we could get some thread sizes and maybe an adapter or four to allow a valve and some tubing to a fluid bottle. Is Nicole flying in?

Ed Scherer

Just think how much easier this would be if we could rent some time on the space shuttle and get Nicole's car into space for a "deep, full body vacuum".


Hmmm... actually, that makes me wonder... is there a such thing as rentable big vacuum chambers where you could vacuum down a whole car? I guess it'd be rather expensive, along with the full suit you'd need while you're in there working on it to inject fluids, close up the bleed screws, etc., before repressurizing it...

Nicole

I still have the old lines, and the calipers - only that one of them is not usable right now.

blown 87

Stupid question, but is every one sure that the calipers are on the right side?

I have seen that more than once, and am pretty keen on looking for that after doing it once myself.

I am sure it is not the problem, but check and see.

I also agree, go back to what worked before and try it to see if you get a good pedal.

brianrheffron

Here is a tip that might be of general help when working on brakes.
I have used this with success on a variety of cars.

Brake fluid reservoirs have a small hole in them to admit air and allow
the fluid level to rise and fall freely. If you open any part of the fluid
system, say unscrewing a bleed valve, there is nothing to stop the fluid
draining out completely. From some of the posts in this thread this
could be a cause of all sorts of brake problems such as air in the ABS unit.

I make up a spare reservoir cap with the air hole sealed using a blob of glue
from a hot glue gun. Before I do any brake work I top up the reservoir to the
very brim then fit the air tight glue-sealed cap. This means that when you open
up the system anywhere fluid loss is reduced to next to nothing.

I have used this method for many years on many different cars.
It seems to work well and and it if nothing else it makes replacing
callipers, brake pipes or flexible hoses a lot less messy.

I have seen other car buffs with older cars that don't have a low brake fluid
float through the cap get the same effect by wrapping the open top of
the reservoir with cling film and screwing the cap back on. I never liked
this way because I was scared of getting shredded cling film in the fluid.

This tip is quick and easy to do and considering the awful problems some
928 owners have with trapped air in the system I think it should become
standard procedure when working on brakes.

dr bob

The fluid reservoir cap has the same threads as a wide-mouth Nalgene bottle I have in the garage. The bottle was a couple dollars at the local plastic bottle emporium. Same threads as the ATF reservoir I suspect. Anyway, instant sealed cap for Brian's method, easy to port with a bulkhead fitting for flush tools, has a lip for sealing built in.