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edlithgow
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PostSubject: Brake Flushing   Wed Dec 03, 2014 5:41 am

Not really a tip, since I havn't tried it. More an idea.

For DIY pressure bleed, any reason you couldn't use a hypordermic syringe, either for downward bleeding, or the other way around? i.e. backflush the system, perhaps after an initial downward bleed?

I'm assuming there aren't any non-return valves etc that would prevent this.

I'm thinking empty and rinse out the reservoir with "turkey baster" style pipette or the syringe, then maybe do an initial low volume downward bleed, perhaps with the stick thingy, (see below) then back-flush it with a hypodermic syringe through the bleed nipple

You'd have to watch you didn't introduce any air, of course, but so do doctors, and they don't kill all that many people, AFAIK.

A syringe is cheap, should give you complete control over the volume delivered or removed, and I'd expect brake fluid would be compatible with its (silicone rubber?) piston seal.
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edlithgow
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PostSubject: Re: Brake Flushing   Wed Dec 03, 2014 6:41 am

Turns out, unsurprisingly, that isn't a new idea

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Apparently pretty tolerant of air entrainment, since he fills the syringe in an unusually kack-handed way which introduces loads of it, but the method still works.

I'd want to do a bit of a downward flush first though, to reduce the amount of crap blown back through the master cylinder.

You could presumably also suck with the syringe, but I suppose that'd increase the chance of drawing air into the system.
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edlithgow
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PostSubject: Re: Brake Flushing   Wed Feb 11, 2015 10:45 am

Havn't driven my car for maybe a month following its suspected master cylinder problem, brake fluid change, and subsequent test drive, which went OK.

In the interim the master cylinder has emptied (didn't leak before) and there's a stain under the rear wheel, so I guessed the bleed nipple was leaking, perhaps because I didn't tighten it enough.

Cleaned it up, applied PTFE tape to the thread, and used a syringe to add more brake fluid via the nipple (a 60ml "enema" syringe and a bit of rubber tubing were 45NT, or about a quid).

Wheel cylinder now leaks. Can't see why changing the brake fluid would cause this, so its either coincidence (best guess) or there's something wrong with the "back flushing" method.

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PostSubject: Re: Brake Flushing   Fri Feb 13, 2015 9:49 pm

What's the benefit of this method supposed to be?
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edlithgow
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PostSubject: Re: Brake Flushing   Tue Feb 17, 2015 1:38 am

I'd guess quicker, less air bubbles, more control, as above.

You'd need quite a lot of experience with multiple methods to confidently state one was "best", and I don't particularly want that much experience, ( though I should certainly be flushing more often than I have been).

However, I am pretty convinced that the "standard" two-person method is avoidable with various simple improvisation, and I think it should be avoided, especially when one is told to push the brake pedal "all the way", which is likely to destroy seals on an older car.

I've replaced the shoes and wheel cylinder but havn't refilled the system yet. I'll use the syringe when I do and see how it goes. I should probably replace the drum too (Linings had both detached and the bare leading shoe scored the surface) but I dunno if I can get one for this car, (especially around Chinese new year) so I'll probably just put it back on.
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edlithgow
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PostSubject: Re: Brake Flushing   Thu Feb 19, 2015 5:09 am

Syringe method seems to have worked quite well. After refilling the system I did a token bleed from the other two (front) nipples but didn't see any bubbles.

It makes intuitive sense that filling "upwards" would be less likely to trap air bubbles. I did, however, have a problem with leaks while filling.

There's a lot of resistance (I believe the flow is limited by a small relief port in the master cylinder) and, with a bit of impatience, you could generate quite high hydraulic pressures with a 60ml hypodermic, which I've read somewhere could blow seals in the master cylinder.

It was difficult to find a degree of opening of the bleed nipple which allowed flow but didn't leak around the nipple, even with PTFE tape on the threads. I also found it quite difficult to tell where the leak was coming from, among the three available candidates (tubing on bleed nipple, bleed nipple itself, and brake pipe union with wheel cylinder) although looking at wetting pattern of applied paper tissues gave useful clues.

It seems possible that using the hypodermic as a passive reservoir (kept just below the level of the top of the master cylinder reservoir) and refilling under gravity alone would be better. The same medical supply place sold enema bags which might be better for this, slightly more expensive but still under 2 quid.

From a quick test drive, pedal is firm, but there's quite a bit of free travel which I'll need to look at.

This may be because I'd backed off the adjuster to put the drums on but was unable to tighten it up through the tiny port in the back of the drum. This might work from a pit, but I found it impossible with the car on fairly low axle stands.

In my fairly limited experience, drum brakes have basically been broken-by-design since they started putting automatic adjusters on them.

1986 isn't really old enough.
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edlithgow
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PostSubject: Re: Brake Flushing   Fri Feb 27, 2015 12:15 am

Draining the whole system (apart from the front brake lines) was an accident this time, but doesn't seem to have caused any grief with trapped air.


(EDIT : Apparently its great grief guaranteed if you have an ABS system, which of course this car does not. ENDEDIT)

If this wasn't a fluke, and If I can stop it leaking at the bleed nipple (perhaps with more PTFE tape and less applied pressure), this could become a hands-off, unattended operation. Leave it to drain under gravity, then leave it to fill under gravity.

Seems to beat all that tedious pedal-pumping.
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