Bleed Screws/Nipples

[Numnut]

Apologies in advance if this a stupid question...

When replacing bleed screws is it best to drain the brake fluid out first, or can I get away with just screwing the new one in?

I think the screw is, er, screwed. When bleeding barely anything came out the screw and fluid just seeped out round the thread and now it weeps a little. Hopefully, a nice new one will sort it out...

[Trinity765]

I rounded off a bleed nipple last year. Replaced it without draining the system and then bled it again. Not sure if that's advisable but it worked for me.

[Yorick]

As the young lady says, no problem just changing it.

[Numnut]

Awesome, thanks very much!

[Mr. Dazzle]

Keep the lid on the reservoir to stop it gushing out

This.

It's like putting your finger over the end of a straw and then taking it out of the drink. So long as you don't break the seal it stays put.

Obviously don't squeeze the lever either! If you DO accidentally brush the lever you may well have to bleed again. At least you'll have a nice new nipple.

[Numnut]

Cheers for the advice - got them changed easily enough. One was pretty rotten...

But...I kept the lid on the reservoir and the fluid tried it's best to escape. The reservoir was almost empty by the time I'd replaced one of them! I stopped the fluid getting on discs/pads/wheels etc.

Does the fact that the fluid wouldn't stay put indicate there may be a problem somewhere? I've got good pressure on the lever and no obvious leaks so I think all is well. There's been quite a few little things with this bike that I don't quite trust it yet!

[weeksy]

gravity

Fluid from the top, leaked out of the bottom.

[Mr. Dazzle]

Does the fact that the fluid wouldn't stay put indicate there may be a problem somewhere? I've got good pressure on the lever and no obvious leaks so I think all is well. There's been quite a few little things with this bike that I don't quite trust it yet!

Probably not.

Going back to the straw analogy, if you try what I said you'll find the water doesn't fall out of the straw. However if you shake the straw about, or if you poke something into the bottom end of it, the water will still fall out. As soon as you 'break' the seal at the bottom end of the straw the air can get up the 'side' of the water and it falls out. Once the water stats falling it all rapidly cascades and everything falls out doesn't it?

When you unscrew the nipple and generally jiggle thing about (which it sounds like you probably had to do a lot of?) you're disturbing the liquids in the system and increasing the chances of air getting in etc.

[Numnut]

Does the fact that the fluid wouldn't stay put indicate there may be a problem somewhere? I've got good pressure on the lever and no obvious leaks so I think all is well. There's been quite a few little things with this bike that I don't quite trust it yet!

Probably not.

Going back to the straw analogy, if you try what I said you'll find the water doesn't fall out of the straw. However if you shake the straw about, or if you poke something into the bottom end of it, the water will still fall out. As soon as you 'break' the seal at the bottom end of the straw the air can get up the 'side' of the water and it falls out. Once the water stats falling it all rapidly cascades and everything falls out doesn't it?

When you unscrew the nipple and generally jiggle thing about (which it sounds like you probably had to do a lot of?) you're disturbing the liquids in the system and increasing the chances of air getting in etc.

That's a good explanation - thanks!

[David]

Keep the lid on the reservoir to stop it gushing out

This.

It's like putting your finger over the end of a straw and then taking it out of the drink. So long as you don't break the seal it stays put.

Obviously don't squeeze the lever either! If you DO accidentally brush the lever you may well have to bleed again. At least you'll have a nice new nipple.

Good theory, but the master cylinder lid needs a breather, otherwise you will end up with a vacuum over it, and that won't end well.
If you need to open the system and don't want to drain, either apply the brake and put a clamp on the flexible, or take off the cap, cover with cling film and refit the cap without tearing the cling film (or a plastic bag will work)

[Yorick]

I've seen it in reverse. Bikes that stopped on track with their front brake jammed on. Simply too much brake fluid in the system.

Take off MC cover and bike moves ok

[Mr. Dazzle]

Good theory, but the master cylinder lid needs a breather, otherwise you will end up with a vacuum over it, and that won't end well.

'tis a very good point! can't believe I forgot that

My straw analogy would only work when the 'top' of the system is sealed, like it is when you've pulled the lever and covered MC inlet port. In the 'off' position you have to let air in so that the space above the fluid in the reservoir doesn't become a vacuum....if you didn't, the brakes would suck themselves off when you let go of the lever, which isn't as fun as it sounds.

That's part of the reason the reservoir has the rubber diaphragm (11) and the diaphragm plate (12) as in this pic. The bottom face of the diaphragm nominally sits up against the top of the fluid and keeps moisture/crap out. The top side of the diaphragm is open to atmosphere and allows air pressure to equalise inside the reservoir.

Just thinking back to the bikes I've owned, I don't think any of them actually have a dedicated vent/breather. They just rely on the fact that the top of the plate (12) and the bottom of the reservoir lid (8) don't make a particularly good seal and let air in where you need it - the plate generally has holes, or fine mesh in it doesn't it. You're not talking very much air.

[weeksy]

Interesting. On Shimano bicycle brakes they have a little hole and I've wondered why before now.

[David]

I've seen it in reverse. Bikes that stopped on track with their front brake jammed on. Simply too much brake fluid in the system.

Take off MC cover and bike moves ok

No...thats because the breather is blocked.

[Yorick]

I've seen it in reverse. Bikes that stopped on track with their front brake jammed on. Simply too much brake fluid in the system.

Take off MC cover and bike moves ok

No...thats because the breather is blocked.

I didn't know you were there. I was

[Tricky]

I've seen it in reverse. Bikes that stopped on track with their front brake jammed on. Simply too much brake fluid in the system.

Take off MC cover and bike moves ok

No...thats because the breather is blocked.

No, it's not- it's as Yorick said , because someone's overfilled the reservoir leaving no room for expansion
Scarily not that uncommon amongst club racers / trackdayers, as I've witnessed more than once, and each time said individual had bled the system the day before, left it with a bungy on the lever overnight, and then topped it right up " just to be safe"

[Mr. Dazzle]

Fluid gets hot and expands, which is why you only notice once you're out on track.

[David]

Fair comment..it does need time to expand and most bikes have a one way breather valve on the m/c. Cars dont fall over so often or so far.
You can achieve the same effect by taking all of the play from the lever/ pedal assembly.

[Druid]

Fair comment..it does need time to expand and most bikes have a one way breather valve on the m/c. Cars dont fall over so often or so far.
You can achieve the same effect by taking all of the play from the lever/ pedal assembly.

I've done this by using a Chinese "pattern" lever on a VFR750. Got a mile from home and the front brake stuck on, removed the lever and it freed off. Five minutes work with a file solved the problem.