Brake Bleed nipple/Mityvac/Brake Fluid query

Apologies for the length of the post; I've got lots of questions

I have bled the brakes a few times now and can get them reasonably firm, reasonably quickly. This weekend, for the first time, I used a Mityvac to fit new front brake hoses. Generally very impressed with the tool. One thing I noticed was in the manual*, it explained that if air appeared to be coming out in the hose in the fluid, yet the lever was hard (hence there was no air in the system) it could be caused by air rushing down the thread of the bleed nipples from "outside" the caliper and then being expelled with the fluid that comes out. I'd noticed this before when bleeding using other devices/methods and had wondered what caused it.

It suggests using Teflon tape around the bleed nipple threads to tighten this up. Am I right in thinking that in order for this to be effective, I'd need to completely take the bleed nipple out (and hence re-bleed the system) and coat the whole thread in the tape in order for this to work?

Alternatively, would getting be bleed nipples be worthwhile (or do "they all do that sir")? I'd hope the caliper is made of harder material, so the wear is on the nipple thread, rather than the female thread of the caliper.

I did the usual things like tap the calipers with a rubber mallet/leave it overnight with the front brake lever cable tied on, etc. The front brake works and is perfectly adequate to be safe to ride with, but it's not great considering the caliper rebuild, new (EBC-HH sintered) pads, braided lines, etc. I changed the front tyre at the same time, so suspect part of the lack of bite may be due to the pads needing to bed in and my reluctance to really yank the lever on hard while riding, but that doesn't account for the lever not being rock hard when pulling it on when stationary. I know the answer is probably to rebleed them to get it firmer, but with this air rushing out problem and the brakes being adequate at present, I'm reluctant to 'gamble' and possibly end up with them working less well.

In terms of the air rushing in the thread problem with the bleed nipples, when using the "syringe from the bottom" method to bleed your brakes, I imagine this isn't a problem? Would you just get very slight seepage of brake fluid from the thread? If so, I may have to look into doing this next time.

One last thing; I know about using new brake fluid and not keeping a half open bottle for months etc. However, I ran out of time to fit the rear brake line so have a whole litre left over which ought be enough to do the rear without buying a new bottle. I'm not expecting anyone to make any guarantees, but what are folks' opinions on using 2 week old (unused) brake fluid, bearing in mind its being stored in a bottle brimmed to the lid and it's only going in the rear brake so a bit of sponginess isn't a problem?


*Indicentally, it was nice to have such a comprehensive manual; so many tools are let down by a lack of clear instructions on how exactly they should be used.

[21QUEST]

Ceri JC,

Don't bother with the Teflon tape. The seal isn't made by the threads....and yes, "they all do that Sir"

The unused brake flide would be just fine. Also, I've used brake fluid that has been older than two weeks myself....then again, I don't always keeps to 'the rules'



Ben

[yorkie_chris]

When you take the bleed nipple out you don't need to re-bleed it, fluid will run straight out of the nipple from the reservoir so so long as you don't let it run dry it be fine. Tie the lever down slightly (so transfer port is closed) and you can open bleed nipple and nothing will happen.

Originally Posted by yorkie_chris

When you take the bleed nipple out you don't need to re-bleed it, fluid will run straight out of the nipple from the reservoir so so long as you don't let it run dry it be fine. Tie the lever down slightly (so transfer port is closed) and you can open bleed nipple and nothing will happen.

So that effectively closes the resevoir->master cylinder of transfer of brake fluid, whilst not applying enough pressure that the fluid in the lines will be pressurised enough to squirt out of the bleed nipple holes when the bleed nipple is removed?

In lieu of Teflon tape, I used bluetac around the bleed nipple (making sure it stayed on the outside of the nipple/caliper!) to create a not quite perfect, but much better seal for the vacuum. This got me thinking, is there some 'proper' equivalent material designed for sealing leaks like this that I could use instead?

One other question re: Mityvac. Is the vaccum pressure on the guage dropping indicative of this sort of leak (well, air being sucked into the system), or is it normal when air is drawn out of the fluid/lines too?

Ta chaps.

[yorkie_chris]

Yeah PTFE tape.

Fluid or air going in will increase the pressure in your vaccuum chamber.

Originally Posted by yorkie_chris

Yeah PTFE tape.

Fluid or air going in will increase the pressure in your vaccuum chamber.

Ah, that's what my old man recommended. I thought it was the same stuff as teflon tape, is it something different then?

[yorkie_chris]

Yup, used for sealing up pipe fittings and stuff, cheap enough. Will stop the nipples from siezing in place too. make sure you only get it on the threads.

[flymo]

I use this tape also, 21_Quest is correct in stating that it does not form any part of the seal when the nipple is closed, but it certainly makes the job of bleeding brakes much cleaner in my experience.

[embee]

Air leaking in at the threads is not a problem when using the mityvac, the air cannot get back into the brake system. Personally I don't bother with trying to seal the threads, it doesn't achieve anything.

As long as fluid is flowing steadily out of the nipple it's bleeding the system effectively. Just keep giving the pump a few squeezes to maintain suction and all will be fine.

I like to do both sides of the fronts a couple of times, swapping between them. I've found the SV brakes will never feel rock-hard, the parts are just too flexible, right from the master cylinder, hoses, through to the calipers themselves. You can get them feeling pretty firm though, and usually they feel a lot better on the road than when you're working on them (it always feels a little disappointing, but try it on the road).

To get the best possible results, allow the bottle of fluid to stand/settle for an hour or so before using it, and pour it very carefully to minimise the entrainment of air bubbles.

It's fine to use fluid from a bottle which has been opened as long as it has had a tightly fitting lid kept in place. You shouldn't leave any brake fluid bottle open to atmosphere any longer than absolutely necessary as it absorbs moisture.

Originally Posted by embee

pour it very carefully to minimise the entrainment of air bubbles.

Entrainment - word of the day toilet paper?