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Completely new here, and completely new to disc brakes! I've just retrofitted a second hand Avid Juicy 3 rear disc brake to a very good steel frame bike using an A2Z MTB disc brake frame adaptor. It's completely solid and works well - no leaks, pistons are smooth and braking is good.
When compressing the lever, there's a good solid point where the pads connect with the disc. This works fine in terms of braking when riding. If I keep the lever compressed it very slowly travels to the fully closed position against the handlebar. It is really slow, and the brake really is solid, but is this right? I bled it really carefully, de-gassed the fluid and got rid of all bubbles, and like I say there's no leaks. What's likely to be the cause of this?
If there are no external leaks, then you have an internal leak. It'll be past the first piston seal in the master cylinder,it may be the seal, or the bore, worn, and the fluid will be escaping back in to the reservoir.
coatesy
Member
If there are no external leaks, then you have an internal leak. It’ll be past the first piston seal in the master cylinder,it may be the seal, or the bore, worn, and the fluid will be escaping back in to the reservoir.
Is this in the lever, or the reservoir?
Many thanks - what you say makes perfect sense and I can see how it would result in what's happening.
One question - does the piston operate within the reservoir? In other words, is there fluid in front of the piston and behind it? So the piston compresses fluid in the pipe and caliper, but has fluid behind it so that the gap behind it when pressed is always filled with fluid?
If that's the case then the leak is just related to pressure in the pipe & caliper reducing slowly but not with fluid leaving the system - which is good to know.
I have a spare lever so will try that. The cover screws are a bit mauled & won't turn so I can't check it, but got to be worth fitting it to give it a go.
Is there a way to check the lever before fitting? eg fill with fluid and seal the outlet?
The master cylinder piston has two seals on it, and the bore has a port into the reservoir just in front of them. When you operate the lever, the first seal passes the port, and creates pressure in front of itself, and on into the hose and caliper. It's second seal never passes the port, and just provides a seal to prevent the fluid in the reservoir from leaking out. You could technically block the outlet of the spare lever to test it, but it would need bleeding first, so you may as well remove the faulty one, and do it on the bike.
So chances are its the first seal that's leaking fluid from in front of the piston back to the reservoir. There's no external leak so I reckon that's what's happening, whether a poor seal or a worn bore.
I get your point, might as well just fit the second lever.
Many thanks!