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Discussion Starter #1
In the past I've had other folks perform the periodic brake fluid flush and bleed. This year I'm doing it myself and have run into something bizarre.

Per the manual I'm bleeding one side front and rear simultaneously. Initially I could do about ten full-stroke cycles of the pedal and then the pedal started returning slowly. After a few more cycles the pedal doesn't return. If I try to push the pedal down further it seems locked. When left alone, the pedal eventually returns to the up position after several minutes. Messing with this for the last few days it seems to be getting worse; it takes fewer cycles to encounter the lock up.

The brakes were working fine before. I'm using DOT 4 fluid.

Any words of wisdom?
 

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1974 GTV 2000
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Just a guess but could it be a vacuum building up in the fluid reservoir? Have you tried it with the cap off the res as you do this?
 

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Sounds like a lot of work, Bob.
I use a pressure bleeder (no pedal pushing), first withdrawing the old fluid from the master cylinder reservoir.
Fill the pressure bleeder with the correct fluid and connect to the reservoir. 8 to 10psi tank pressure is enough.
Start with the right (passenger side) rear brake and bleed until you see clean clear fluid.
Next would be the left (driver side) rear brake, followed by the right front and left front.

Same procedure for the clutch, as well.

The bleeder that I use has the correct screw-on fitting for my reservoir.
MVP-0090_ml.jpg
 

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Discussion Starter #4
NZRossi: Good idea! Alas, it made no difference.

74 GTV: I have speed bleeders so it's really quite simple (if it would only work). Do you not have to bleed front and rear at the same time with the pressure bleeder? I would have thought that the rear brake differential pressure thingy would have not permitted one-wheel only bleeding.

PS: Thanks to both for the thoughts.
 

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Sorry to hear about you brake issues, Bob.
The unit I use has been working great for many years. See link... Collections
I personally do one brake at a time, although I've seen some dealerships (not Alfa Romeo) do all at once, letting fluid run on the shop floor. With that, someone else must cleanup the mechanics mess.
Again, I've never experienced any issues with bleeding one brake at a time.
 

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On another recent thread, one piece of advice was to put a stop under the brake pedal to prevent a full push of the pedal to the floor. Logic was that a full push pushes the M/cyl components into ‘uncharted‘ territory. This may well be what you are experiencing. Tho I’ve never had this happen to me, I see some logic.
 

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Discussion Starter #7
Ranz: Gee; that sounds suspiciously like what I may have done. I wonder if there's a fix or technique to pull the master cylinder components back into "charted" territory.
 

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But Mad North-Northwest
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If you've got the bleeders open the pedal won't really return on its own very well, as there's no pressure in the hydraulic system to push it back. You usually need to manually pull it up after each stroke. Are you saying you can't do that?

All the Alfa manuals say to bleed both bleeders on one side. I've never, ever done that and have not had any problems. The only reason I can think of is maybe it's a holdover from the cars that had a differential brake pressure warning valve? But even on the '74 that has one of those I've bled each wheel individually without issue.

If you've pushed the brake MC into uncharted territory it's very possible that you may have damaged it. The issue is that any rust or gunk on the cylinder in that unused area gets picked up by the piston seal, and then the seal is well and truly f-ed. Ask me how I know this.

I've switched to a Motive pressure bleeder and have never looked back.
 

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When left alone, the pedal eventually returns to the up position after several minutes.
Stooie said:
I wonder if there's a fix or technique to pull the master cylinder components back into "charted" territory.
Stoo:

Good to see you are still working on the Alfa - haven't checked in for awhile.

I've never experienced this "uncharted territory" problem, but don't have a better explanation for what's going on. Two ideas:

- Perhaps letting it sit for several minutes, and the pedal returning to the top, IS the technique to put things back into "charted territory". Perhaps your next step should be to put a block under the pedal, as Ranz advises, and see if you can complete the bleeding with pedal lock-up. If so, and the pedal feels firm, then all is probably well.

- I have always bled one wheel at a time, as 74 GTV advises, starting at the LR (longest line from the MC). By "the rear brake differential pressure thingy", do you mean the device on the L firewall with an electrical connection? Or the rear pressure limiting valve? I don't think you can trigger the differential pressure sensor just by bleeding, since the two circuits don't build much line pressure when bleeding. After all, there is negligible pedal pressure when bleeding.
 

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Discussion Starter #10
Jay:

Howdy! I hope all is well in SJC.

In this case the "thingy" is, indeed, the rear pressure limiting valve. The owners manual (how cool is it that the owner's manual devotes two full pages to brake fluid flush and bleeding?) advises to do a front and rear simultaneously as with just one bleed port open, say on a rear wheel, one won't be able to depress the pedal 'cause the front wheel circuit is still closed.

Perhaps for either pressure or vacuum bleeding one might have a chance doing one wheel at a time.

Gubi: Thanks for your insight. Yes, I can pull the pedal pack up manually.

I'm getting a bad feeling that a new master cylinder may be in my future. They're available and not terribly expensive, but I can't imagine changing one without dribbling paint-damaging brake fluid all over the place. Ugh
 

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But Mad North-Northwest
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In this case the "thingy" is, indeed, the rear pressure limiting valve. The owners manual (how cool is it that the owner's manual devotes two full pages to brake fluid flush and bleeding?) advises to do a front and rear simultaneously as with just one bleed port open, say on a rear wheel, one won't be able to depress the pedal 'cause the front wheel circuit is still closed.
]

The differential pressure valve in the rear just limits pressure to the rear brake circuit. It does not affect bleeding since you're nowhere near the pressure limit while bleeding, and it's not the reason they tell you to bleed the front and the rear at the same time.

I've done it one wheel at a time by depressing the pedal too: it worked fine.
 

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The only thing I have to add to this is that I found I was completely unable to use the motive pressure bleeder - which I consider to be one of the best tools in my garage - with the factory reservoir because of air leaks. I cannot recall where I got this one from most likely classic alfa I believe it is the style used on later year cars but it has only one cap and is easy to get good pressure on.

IMG_1716.jpg
 

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The differential pressure valve in the rear just limits pressure to the rear brake circuit. It does not affect bleeding since you're nowhere near the pressure limit while bleeding, and it's not the reason they tell you to bleed the front and the rear at the same time.
I assume the reason the manual tells you to bleed the front and the rear at the same time is that other thingy: the sensor on the right firewall that detects when one brake circuit has more pressure than the other. But as Gubi says, you can bleed one wheel at a time. Unless you really stab the pedal while bleeding, I doubt you would trigger that sensor.

Stooie said:
I'm getting a bad feeling that a new master cylinder may be in my future. They're available and not terribly expensive, but I can't imagine changing one without dribbling paint-damaging brake fluid all over the place.
There will never be an easier time to change the MC than when you have to bleed the lines anyways. It doesn't have to be that messy - begin by sucking all the fluid out of the old MC reservoir. Put a lot of newspaper, plastic, whatever beneath the MC to catch any drips. And of course, wipe off any spilled brake fluid immediately.

I hope all is well in SJC
It is, thank you. Kind of quiet these days. We missed our annual spring visit to Salem and Corvallis this year. Hope to do it again soon.
 
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