Shimano Hydraulic Brake question....

[Nigeyy]

So recently was installing some new Shimano lower end hydraulic brakes. As I'm paranoid about over tightening things, I shortened the hose, put in the joining insert and olive and screwed it in, but trying not to over tighten it.

Well, the brake lever would initially hold pressure, but then slowly you would feel the lever go all the way to the handlebar. No problem, I thought, I'll just bleed it, so I did...... 3 times over with the same result (should add, I'm experienced at bleeding Shimano brakes). At this point, I was really considering it was a bad brake with seals letting fluid go past -I mean, no brake fluid was leaking out anywhere, and the lever was going to the handlebar after initially feeling firm.

It was then I noticed that the amount of threads left visible where the hose connects to the lever assembly compared to my other brake was considerably more, so with much caution, I tightened the hose even further.... and voila! The brake was back to normal, perfect!

So my question is: how did tightening it fix this issue? When I depressed the lever, wouldn't I expect fluid to be leaking out? The complete absence of brake fluid leaking out anywhere really made me suspect the brake itself was bad -and without a shadow of a doubt, tightening the connection completely fixed the issue. I'm trying to visualize how the brake works -I always thought in hydraulic brakes increased pressure in a master cylinder to the activate the slave cylinder was the way it worked -and as such, if there was a poor connection, fluid would leak out allowing for the piston to depress, much like what I was experiencing.

Anyone with any explanation as to why this is?

 

[Cycleops]

Certainly no expert in bleeding bike brakes but maybe it was pulling air in and viscosity of the fluid meant it didn't leak out?

 

[Tom B]

I use M615 brakes and Levers.

Recently, after a few years, I noticed one was getting closer and closer to the bar. Inspection showed the tiniest amount of weepage behind the dust caps on the Levers. I nipped up the nut pressing on the olive and refilled and all was well. The amount of fluid lost was miniscule but it made the difference.

I tightened the nut really quite tight, but with the squishy hose and olive to be squished I expect there is some force required to cause the squish, make the seal and keep in the fluid which can itself see some pressure. The levers seem pretty stout.

 

[Yellow Saddle]

There are three possible solutions to this:

1) The laws of physics were temporarily suspended as you finished that beer and observed the magic.
2) There's a leak that you have not yet found.
3) The first few "pumps" merely advanced the pistons towards the discs and since no real pressure was behind them, the hose ends didn't leak.

The amount of fluid that's "pumped" with each push of the lever is small - one drop small. Still, it must be there somewhere.

Just to make sure it wasn't the beer but option 2, remove the flare nut carefully and look for oil in the threads.

 

[lpretro1]

It didn't work because you hadn't tightened it up enough to form a seal by compressing the olive against the insert. Fluid will have been leaking out - you just didn't notice it. With the Shimano ones you do have to tighten then quite hard

 

[Tom B]

flare nut.

That's the word(s) I was looking for.

 

[Yellow Saddle]

That's the word(s) I was looking for.

I accept PayPal.

 

[Nigeyy]

Just saying... but:

I definitely checked to see if I could see fluid leaking when it wasn't working -at the calipres, at the lever, and I did this at least 3 times. I'm pretty certain, I didn't see any leakage (wiped clean with a rag, used a light to see any shininess and my finger tip to test for fluid). Also, thinking about the number of times I squeezed the lever and it gradually going back to the bars, I reckon there would have had to be a noticeable amount of fluid.

I'm really afraid there is another leak somewhere, but strangely the brake feel just as good today as they did yesterday..... At this point, I admit to being very perplexed.

 

[lpretro1]

Well it can't vanish into thin air - you fettled the hydraulic hose - not any other part of the system - and it became a problem - would suggest what you did was the issue. It may just have been tiny amounts coming out - there isn't a lot in there anyway