Dellortoman
Hero member
- Location
- Tasmania, Australia
I've often heard of tying the brake lever on overnight to get rid of air in the system. Like Carlo in his Jota thread. https://www.laverdaforum.com/forum/index.php/topic,95568.0.html
Heaps of others have either done it or suggested it as a method of "bleeding" air out of the system, so I'm not singling Carlo out. It's just that his comment about doing it is the most recent.
It's a complete myth, at least in the way most people think it works. Like this bloke for example.
He says that tying the lever on allows air bubbles to float to the top, TO A HOLE WHERE IT CAN ESCAPE. That is total bollocks. If that were true and there was a hole venting the system to air, then there would be no pressure in the system and the brakes wouldn't work at all. This kind of pseudo mechanic bullshit is about as helpful as astrology to anyone wanting to understand their machine.
The bloke in the above video is not on his own. They're all over youtube. There was even one bloke who tied his rear brake pedal back to the footrest, where the pedal pivot was, so it didn't apply the brake at all. ??? This kind of misinformation pisses me off, which is what motivated me to start this thread.
The only time the system is open to ambient air pressure is when the M/C piston is in the rest (fully retracted) position. The rubber seal on the piston moves back past the wee little port from the fluid reservoir, opening it to the hydraulic system. The purpose of that little port is to release any residual pressure (so the brakes don't stay on) and to allow fluid to flow into the hydraulic system if there's any loss of fluid volume. But as soon as the brakes are applied, the master cylinder piston moves past that inlet port in the cylinder, and the hydraulic system becomes closed. A moment's thought will tell you that this must be so, otherwise there's no way it could build up pressure. Nothing can get in or out of a closed system. There's no way air can get out unless there's a leak somewhere, and even then, to be of any use, the leak would have to be exactly where the air bubble is. Anyway, if that's the case you have a leak that needs fixing.
If you use this method and it makes the brakes less spongy (as many people will attest) then it's not because of air escaping. Something else is going on. I'm not sure what, but one possibility is that the pressure in the system is stretching the rubber component of the hydraulic hoses until the reinforcing fabric or steel braid takes up the load. Rubber is not an elastic material, so it doesn't spring back fully when the load is released. What you end up with is a pre-stressed brake hose that has very little "give" so it makes the brake feel firmer.
However, if the above explanation is true, then the effect won't be permanent. The rubber will slowly recover and the sponginess will return in time. I wouldn't mind betting that anyone using this method to "bleed" brakes will find that it's only a temporary fix.
Please feel free to disagree and shoot holes in my argument.
Heaps of others have either done it or suggested it as a method of "bleeding" air out of the system, so I'm not singling Carlo out. It's just that his comment about doing it is the most recent.
It's a complete myth, at least in the way most people think it works. Like this bloke for example.
He says that tying the lever on allows air bubbles to float to the top, TO A HOLE WHERE IT CAN ESCAPE. That is total bollocks. If that were true and there was a hole venting the system to air, then there would be no pressure in the system and the brakes wouldn't work at all. This kind of pseudo mechanic bullshit is about as helpful as astrology to anyone wanting to understand their machine.
The bloke in the above video is not on his own. They're all over youtube. There was even one bloke who tied his rear brake pedal back to the footrest, where the pedal pivot was, so it didn't apply the brake at all. ??? This kind of misinformation pisses me off, which is what motivated me to start this thread.
The only time the system is open to ambient air pressure is when the M/C piston is in the rest (fully retracted) position. The rubber seal on the piston moves back past the wee little port from the fluid reservoir, opening it to the hydraulic system. The purpose of that little port is to release any residual pressure (so the brakes don't stay on) and to allow fluid to flow into the hydraulic system if there's any loss of fluid volume. But as soon as the brakes are applied, the master cylinder piston moves past that inlet port in the cylinder, and the hydraulic system becomes closed. A moment's thought will tell you that this must be so, otherwise there's no way it could build up pressure. Nothing can get in or out of a closed system. There's no way air can get out unless there's a leak somewhere, and even then, to be of any use, the leak would have to be exactly where the air bubble is. Anyway, if that's the case you have a leak that needs fixing.
If you use this method and it makes the brakes less spongy (as many people will attest) then it's not because of air escaping. Something else is going on. I'm not sure what, but one possibility is that the pressure in the system is stretching the rubber component of the hydraulic hoses until the reinforcing fabric or steel braid takes up the load. Rubber is not an elastic material, so it doesn't spring back fully when the load is released. What you end up with is a pre-stressed brake hose that has very little "give" so it makes the brake feel firmer.
However, if the above explanation is true, then the effect won't be permanent. The rubber will slowly recover and the sponginess will return in time. I wouldn't mind betting that anyone using this method to "bleed" brakes will find that it's only a temporary fix.
Please feel free to disagree and shoot holes in my argument.
