Master cylinder brake fluid question
- Thread starter FattBoyy
- Start date
TK4
Well-known member
It will drop, but not any kind of noticeable amount. What's the nature of your concern ?
It should not (appreciably) change. The first thing that happens when you start pushing in the piston in the master cylinder, is that it seals off the vent port that otherwise connects it to the reservoir.
A wee, tiny bit of fluid goes out the vent port until the piston travels far enough to seal the port - so theoretically, the level in the reservoir goes up when you squeeze the lever, but the amount is insignificant.
A wee, tiny bit of fluid goes out the vent port until the piston travels far enough to seal the port - so theoretically, the level in the reservoir goes up when you squeeze the lever, but the amount is insignificant.
FattBoyy
Well-known member
My brake fluid goes up when I pull the brake lever.
But the reason I am looking into this is. My brakes are real strong and work great BUT if I squeeze it real slowly, like how you would if you were going 5 km and had to stop then the lever goes all the way to bottom right to my knuckles almost.
Sound like the seal is shot to me
FattBoyy
Well-known member
You mean piston seal right? But if the seal was done, would there not be brake fluid leaking? Can you explain what you are saying is happening and why does it only happen when I pull lever real slow
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There are multiple seals on the plunger on the M/C. The one that actually produces pressure will normally have brake fluid on both sides of it - if the seal is worn or damaged, when you squeeze the brake lever, some brake fluid will escape the pressure side and flow back into the fluid reservoir.
Can't say why it only happens when you squeeze it gently - maybe the rapid pressure buildup is expanding the seal? The seal is conical and the larger end faces the pressure side
Can't say why it only happens when you squeeze it gently - maybe the rapid pressure buildup is expanding the seal? The seal is conical and the larger end faces the pressure side
FattBoyy
Well-known member
Found this, I have never been in a mc, is this a hard rebuild?
Depends on your skill level, and also if you have the correct tools. Some people dont even own a pair of snap ring pliers, so in that case its kinda hard if you dont own the tools your going to need.
Have you done brakes before, are you good at bleeding?
Ive seen your issue from debris getting stuck on a seal causing it to have blow by. Could be a worn out seal, or it could also just be dirt or debris on a seal. Sometimes it can be fixed with a simple tear down cleaning and clean reassembly
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FattBoyy
Well-known member
I have rebuilt calipers and bled brakes on bikes but never opened up the master cylinder.
So debris gets stuck inside those tiny seals in the picture... So most like this is not a caliper issue at all?
Not that I would definitively say this or that without actually taking things apart myself, but if the symptom you described was a caliper issue you would probably be losing brake fluid
8mm (probably) box end wrench. Do not overtighten those!
Put an old rag underneath the area and around the end of the bleeder screw, then while gently squeezing the brake lever, loosen the bleeder screw and immediately snug it again. If there is a piece of dirt or corrosion preventing it from sealing properly, this should move things around and let it seal.
It would take a substantial leak (not just a tiny weep) to allow the brake lever to come back to the handlebar. I'm thinking you have a seal problem inside the master cylinder and/or contamination with a piece of dirt.
Put an old rag underneath the area and around the end of the bleeder screw, then while gently squeezing the brake lever, loosen the bleeder screw and immediately snug it again. If there is a piece of dirt or corrosion preventing it from sealing properly, this should move things around and let it seal.
It would take a substantial leak (not just a tiny weep) to allow the brake lever to come back to the handlebar. I'm thinking you have a seal problem inside the master cylinder and/or contamination with a piece of dirt.
FattBoyy
Well-known member
It was 8 mm . Did what you said and stopped the leak. Now I guess I have to rebuild the master.. Hopefully there is YouTube videos on it
Caliper seal leaks would usually result in fluid loss. Unlike a master leak where you could be loosing pressure without any actual guild loss. As the fluid is blowing by an internal seal but remains in the system as there are other seals not allowing to actually leak out of the system.
Clean and inspect everything, but for me generally a caliper seal leak is usually noticeable either by the mess of fluid leaking out onto the calipers, pads, rotors and wheels. And substantial fluid loss due the the level in the reservoir going down
I wouldn't rule anything out without cleaning and inspecting though. Caliper seals can and do leak, for the same reasons as the master seals. Could be worn out, or just dirty/debris casing a seal to leak
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When you pull the lever slowly, the fluid has the chance to bypass around the leaky seal without ever building pressure. Normally, the pressure helps push the seal tightly against the side. When you pull quickly, the pressure builds faster than the fluid can bypass the seal, which ensures a tight seal.
FattBoyy
Well-known member
Makes sense
