I have a few bikes with shimano disc brakes. I know changing the fluid is something that can be done and I could probably work out how to do it but I don't know what to look out for to know its time to change the fluid. I know someone who took in their 8 month old mountain bike to the store for a service and they said the fluid needs changing. Is this too soon? Are there any changes I can notice in my brakes that will let me know its time to change the fluid?
The suggested interval to bleed your brakes seems to different among manufacturers.
While SRAMs techdocs explicitly mention to replace the fluid every year, Shimanos techdocs (p20) does only recommends to replace the oil
if it becomes severely discolored. For recreational use, I think you're safe bleeding once every two years but depending of the usage and storage it might be quicker.
Magura on the other hand claims their Royal Blood oil (p21) does not age. I've ridden an MTB having HS33 brakes with 10 year old oil and indeed had a firm braking experience.
Are there any changes I can notice in my brakes that will let me know its time to change the fluid? Replacing the fluid before the regular maintenance can be required if air got in the system. Magura suggests to replacing the oil if:
- the brake does not respons immediatly when the brake lever is actuated
- pressure point is not clearly defined, it is spongy or does not remain constant
- the brake hose has been changed
If you have multiple bikes and like to do some own maintenance, it's even worth looking into buying your own bleeding kit. Shimano has nice technical documentation and there are various videos online giving you a feeling how to bleed your specific brake type.
Small sidenote, regarding storing your bike: ideal you put the flat and upright, so there is less chance for air coming in the brake system. While transporting the bike be sure not to press the brake levers without having the wheels in your bike, as it can cause the pistons coming out too far.
It is common in the automotive industry to change the fluid when problems arises. They have the longest experience in the field and that's what I do. Change the oil when the brake lever feels spongy or less firm.
One trick to have the firmest lever possible is to block the lever with a cable tie overnight, making sure that it is in the highest position, so that all the air can go easily toward it(bike in oblique or vertical position, it depends on your routing).
In the mtb world some people will change fluid every 6 months to 1 year, this is a good practice if you race.