Hydraulic brake problems

[scarry]

I have tektro auriga hydraulic brakes on my bike.

I have had persistent issues with the rear brakes.I have to bleed them every three months or so,and then they are fine.
Then they start to get spongey again and eventually fail completely.

They were fine for the first 4 yrs or so.Fronts are also good.SWMBO has them on hers and they have been good as well.

Took it to the bike shop and they weren't much help.

Why do they have this issue,can't see a leak anywhere either?

They must be getting air in somewhere?

Thanks in advance.

[Marmoset]

It might be some dirt on the caliper pistons interfering with the seals. Take out the pads and extend the pistons out (not too much though or they pop out altogether) and then take a toothbrush and either use some bike specific disc brake cleaner or whatever brake fluid they use and give the sealing surface of the pistons a brush to see if it will clean it up a little more.

I've managed to solve a similar issue by doing this. If that doesn't do it have a really good look at the piston at the lever, if there's a dust boot over this then lift it and take a look, as it may be covering a slight leak.

Sadly, things like this are more economical to fix by buying a new brake set than sending off for repair....