Leaking Shimano pistons?

Excellent, brakes working as intended, please buy a new caliper - Yozo Shimano.

Give it a bleed just to see. Check the pads carefully for mineral oil to begin with - you often see it on the backs. If it's there then the caliper is for the bin. You can get aftermarket seal kits, but at half the cost of a caliper it's more of a project for folk who like fixing things.

Bleed first in case the air in them has contracted due to cold and the squeal is just a bit of road grime from damp surfaces…?

Take pads out and have a close look a the pistons, if no sign of fluid. Bleed them and don't forget to rotate the lever back and forward and adjust the reach screw. Can't believe how many more bubbles came out.
Then zip tie lever to the bar overnight and see if the bitepoint is the same.
I was given these tips on here.
Ended up buying a new caliper in the end as one of the seals was leaking but it was good practice going through the bleed process as it's the first time I'd bled my brakes.

Thanks all. I did a quick lever bleed after first noticing it but think you're right, full bleed (especially if no signs of fluid leak) should be the first thing to try.

the brakes are squealing for the first few uses.

If you've got contaminated pads, I think they would continue squealing and power would be noticeably affected.

Certainly that was the situation when I had this problem on Tiagra calipers.

While not discounting other solutions, one thing guaranteed to make my five year old Shimano brakes squeal is....salt. Horrendous squealing on two occasions when salt spray drifted over the bike during coastal riding and...yep, during the recent cold spell. Required prolonged, hard braking to burn it off. Don't write off the caliper just yet.

The reason I thought fluid rather than salt etc was it's just the front having an issue. The rear usually takes more of a battering with potential contaminants.

Will investigate further and try a full bleed anyway.

Bit of a hijack

Is there a check for leaks that can be done in a garage. I'm suspicious my road bike calliper is leaking but it is out of warranty. It wasn't quite as powerful as it should be and had a tendancy to howl in the wet - this got worse at one point so it hasn't always been like that. Before I drop it off for a replacement I wanted to check. Summer bike, in the spare room so unlikely to be ridden until the sun comes out and the roads dry up. Just had a look but other than a bit of brake dust, I'm not sure there was anything obviously oily on the calliper/pads.

Most other things I check on the rollers, not sure on my skills to test a front brake!

To actually help the OP. When I had problems on my mtb. They went away when the calliper got wet. Not sure why but well aimed jet of water from my bottle improved things. It dried out and then it lost power again.

The obvious check for me was when I replaced pads the day of a ride. Great for a bit then the problem came back, byt the second ride it was awful. Cost a set of pads though :(.

It'll be either the transfer port, or one of the piston seals. Both of which are available aftermarket if you hunt about a bit, although not from shimano.

I've successfully solved 4 transfer port leaks with new O-rings, but I've not had an actual piston seal leak to test the new ones I hold in stock. Yet.

I've also had leaky seals on a Magura caliper, at least I could buy a seal kit for it. Although I found a new caliper with pads for not much more, just like Shimano 🤷‍♂️

Come to think about it, my daughter's bike has a failed Shimano caliper which I have a replacement caliper ready to get fitted (complete with new pads). Might do it this soggy afternoon now it's on my mind 👌