I replaced the fluid in my hydraulic disc brakes as they would not work within the first 10 minutes of cycling and would gradually sharpen up. I suspect they had air in the system.
They were sharp for about 2 weeks after replacing the fluid, but now my front brake has almost no stopping force.
How do I determine the cause of the issue?
Thanks
Tracking down issues with brakes can be difficult. If the brakes are spongy feeling initialy and pumping them firms them up then you probably have air in the system. if that's the case then it will be a bad bleed or the seals or possibly the brake hose but working out which is often down to luck. If they just have no power it could be contaminated pads.
If it was me then I'd do the following, give the system are realy good clean and finish off with disc brake cleaner if you have some. Next I'd re-bleed the brakes (having first pushed the pistons right back) and then fit new pads. Use the brake somewhere safe and then inspect everything to see if you can spot any leaks at the levers, hoses or calipers, take the pads back out and check for contamination with brake fluid.
If you do have a leak then you'll need to replace the seals and/or hoses that have failed - this can be tricky if it the caliper seals that have gone. Getting hold of spares can also be an issue.
It may be easier and cheaper in the long run to replace the brakes. Shimano Deore are relatively cheap and very efective.
Personally I'd (initially) try taking the top off the reservoir in the lever (while it is level!) and pumping the brakes slowly, topping up the reservoir if necessary, then pumping some more while tapping the brake itself then tapping the hose from the brake to the lever.
If you top up he fluid in the reservoir too much while the pads are worn you will need to take the reservoir cover off and surround it with tissue paper when you want to push back the pistons when you replace the pads!
Cover the rotors! You don't want any brake fluid on them (it is not actually as fatal to the rotor as many insist, but it is annoying.)
If this fixes it but a short while later you have issues again then you probably have a leak somewhere.
If you do actually have a leak then it may be at the brake itself given the way you used to get braking power a while into the ride, it could just be the rotors/pads getting a little hydraulic fluid on them from a very slow leak between rides and not air as you thought (though equally it may have been air) you could check this by holding your brakes and pedalling somewhere safe and seeing if power begins to return as you burn off whatever is on the brakes (this can take a while with hydraulic fluid.)
When I contaminate my rotors and/or pads I tend to dump a bit of crud into the brakes and smudge it on to the rotors on purpose, before pedalling with brakes on somewhere safe, unlike iso, the crud option is always in plentiful supply out on the trail :) (Mid ride beer & burger stops are the cause of most of my rotor contamination issues...)
If you have leaks at the pistons themselves I would agree with Jackson and second his recommendation of replacing the brakes with cheap but solid and reliable Shimano Deore.
If the brake levers feel solid when applied then your pads need replacing. Otherwise there is air in the system and they need bleeding. The only other possibility is that your pads may be contaminated (with oil) in which case the brakes will appear to work but not slow you down properly - replace the pads.
