I’m dusting off my bike, which has been left in the shed for a few years without being ridden. The rear brake doesn’t have any power, and feels spongy at the lever. I tried bleeding it with some mineral oil that has been in the shed for about 5 years, which made a small improvement, but hasn’t fixed it.

Is it possible my mineral oil has gone bad? Or am I just crap at bleeding brakes?

I have also replaced pads and disks btw.

Not the oil itself but the additives in it can go off over time.

It’s just a fluid to push the piston. Crap brakes are normally due to air in the system, sticking pistons or bad pads or pad/disc contamination but the last two we can rule out.

Seals are the most likely culprit, could be dried out or even perished.

Worth taking them off the bike, take the pads out and clean up around the pistons without squeezing the lever. Then put something inbetween the pistons that wont damage them but will stop them popping out fully and squeeze them out a bit with the lever. Clean up around them with a cotton but and then use either a bit of mineral oil or some silicone lube on the outside of the piston where it slides on the seal. Push them back in and clean up any excess oil/lube.

Next check around the plunger shaft from the lever and see if you can clean up and lube around that.

Then do a full fluid flush and re-bleed.

Ah thanks andyl, I wouldn’t have thought of the seals. I’ll try as you suggest.

Tbh it absorbs moisture so there’s a good Chance its done for

mineral oil does not absorb water like dot fluid.  My guess would be dry seals

It’s Dot 4 that absorbs moisture.

When you refit them check the pistons at coming out nice and evenly and make sure you align the caliper properly. Makes a hell of a difference having both pads contacting the disc correctly.

Seals are aged?

I’ve just done the DIY transfer port seal swap out on my shimanos, and during the research I found that typical buna 70/90 has a shelf life of 7 years. Yours have sat for at least 5 years in the shed.

They’ll go brittle which could mean leakage of oil and intake of air.

It’s Dot 4 that absorbs moisture.

And 5.1.  But it still works when it’s absorbed all the water it can, you just get a lower boiling point.  It’s still quite high though even then.

apologies my mistake. I just looked at brakes not working an old fluid 🙂

Has anyone else used vegetable oil to bleed their Shimano brakes?

(higher boiling point than olive oils obvs)

Some interesting links

http://www.epicbleedsolutions.com/blog/dot-brake-fluid-vs-mineral-oil/

https://bikerumor.com/2013/04/11/tech-speak-brake-fluid-break-down-and-implications-for-road-disc-updated/