Bubbles in brake reservoir

[eburrows]

I have to confess: I made a major bone-head mistake. While bleeding the brake system on my 93 spider, I let the rear reservoir go dry.

So, after filling the reservoir back up, and starting the bleed process again (starting with the right-rear, where I ran it dry, then planning to redo all four, starting at left-front), I couldn't get it to run cleanly. Just kept getting air out of the bleed screw of the caliper.

Watching while my wife worked the brake pedal, I saw air bubbles coming up into the reservoir from the bottom, as the pedal was allowed back out. After running two liters (!) of fluid through the system, same behavior: Air out of the bleed screw, and bubbles into the reservoir.

Could I have ruined one of the seals in the master cylinder, and now it's leaking air? Any other ideas where those bubbles might be coming from?

I'm thinking about retrying with a power bleeder, but that won't fix a broken seal!

Thanks,
-Erik

[DonBeeson]

How long did it go dry for?

To give you an idea of whats happened to me, I've had a clutch pedal fall to the floor (by its own will) during a transaxle swap. Lost its fluid completely for at least 2 days, and there is still no leak from my clutch system.

I would say to try vacuum bleeding and start at the front. If the system is sealed then the only area you will get air is open bleeders, or air that has made its way into the system!

[usatrade]

Hi
Start the engine and bleed with engine at iddle (in well ventilated area).
Claude

[usatrade]

Hi again,
I meant bleed using the usual method but with engine iddling.
Claude

[eburrows]

Quote:

Originally Posted by usatrade Hi Start the engine and bleed with engine at iddle (in well ventilated area). Claude

[Tifosi]

Likely the bubbles are coming from the MS running dry and it's trying to re-prime itself the only way it can: by purging air up through the resevoir.

[usatrade]

Hi
I found out bleeding my MS in my 79 spider to be and endless procedure. the MS would constantly have bubbles, I would habe bubbles at the end of the line on the right rear brake caliper. Until i started the engine and let it iddle than bled again. All went back to normal in 3 pedal pumps. I do not know why but this is a fact. Exact same on left rear caliper then front.
Does not cost anything to try.
Before starting tjhe engine i went through 2 quarts of brake fluid. After startiong engine just filled up the MS twice.
Claude

[Bob Sacamano]

By a wide margin, the best method I've ever used to bleed hydraulics (on maybe 10 different cars) is the Motive Products pressure bleeder. I've got an early S2 Spider, a late S3, and a late S4, and I can do a complete bleed on any of them in maybe 15-20 minutes, unassisted (it takes longer to get the wheels off than it does to bleed). That includes even a dry system, as on the S2 and S3 I've installed all new hard and flex lines. Sacrificing originality, I even replaced the brake fluid reservoir on the S2 with the single cap type used on later cars, just so I could use the pressure bleeder.

Scroll down to model number 0109:

Products Bleeders

This is one of those tools you'll smack yourself for not buying sooner.

[eburrows]

Thanks guys! I'll try the engine-idling technique tonight, and if that doesn't work, go the power bleeder route.

[Bob Sacamano]

Can't imagine what idling the engine during brake bleeding is supposed to do. Yes, it will provide a vacuum for the booster, but all that's going to do is take the "feel" out of the brake pedal. Without the booster in play, your brake pedal should have a minimal amount of free travel (after a proper bleeding), 1/2" perhaps, and then it should feel like it's part of the frame (i.e. very solid). What is idling the engine supposed to do, Claude?

[usatrade]

Hi
I had exactly same "bubbling" problem for hours we tried to bleed NO WAY Bubbles bubles and more bubles even after filling the caliper as well.
then for an unknow reason I thought of starting the engine and let it iddle before pleding again. Thrre stroke on the pedal, no more bubbles. All four calipers were bled in no time flat and the pedal hard as a rock.
So I do not have a technical expalnation to give you but it worked for me this time (I have bled brakes in zillion different cars, never had the bubbles bit)
As I said it is worth trying as it does cost a penny to try.
I have to redo the calipers on my other 79 Spider soon so i will try that again.
Claude

[Bob Sacamano]

Interesting. I wonder if just the vibration of the engine causes the tiny bubbles to coalesce into larger ones that are easier to bleed out?

[eburrows]

Well today I tried bleeding the system with the engine idling, but that did not seem to help. I got a mityvac bleeder, and used it successfully to bleed the brakes. I did have to adopt another suggestion I found on this board: Wrapping the bleed screw threads with some teflon tape. Until I did this, the mityvac was just pulling air by the threads, and I was just getting bubbles.

I wonder if this was in fact my problem all along. Maybe while I was bleeding before, on the return stroke of the brake pedal, a small suction was formed, sucking air back into the caliper by the screw threads, then pushing it back out on the next stroke...

Unfortanately, the brake pedal feel is a mushy as ever, though not worse than before. The pedal goes in a good inch before much resistance is felt, but is then nice and hard and responsive. I'll bleed the system again in the next couple of days, and also look for any slop in the linkages.

I appreciate your help guys!

[vf31rhill]

After you get into a bleeding frenzy you end up with frothy fluid in the system, tiny air bubbles everywhere. Let it rest for a day or two and then try again. Time will let the tiny bubles collect into a few large bubbles and when you bleed these out you will have your nice hard pedal again. The inch of mush is compression travel to compress the frothy mess down to system pressure and then it acts correctly as everything in the system is at equal (solid fluid) pressure. Let up on the pedal and the froth expands and next foot of the pedal produces same 1 inch of mush. I agree with Bob Sacamano that idling the engine should not make any difference. But if you don't asphyxiate yourself in the process, what harm can it do? If you had good brakes before you made your mistake and let the reservoir go dry, then there is no reason to suspect the vacuum booster.
Robert

[Bob Sacamano]

I used to have a vacuum bleeder, and never found it to do a sufficient job. Just a guess, but I don't think it moved the fluid fast enough, and between strokes of the pump, the bubbles rose back to a higher level, and just get moved back and forth without being flushed out. The pressure bleeders provide a quick, continuous flow, and the bubbles don't stand a chance. I agree with Robert about letting the system rest for a while before having another go at it. Buona fortuna (good luck).