I have a braided line set coming soon, and in preparation for that I did a pre-emptive check of the front brakes to see what condition the pads and such were in.

Removed the pads from the front right caliper, and must've pulled on the front brake a little too much while checking the piston as one of the pistons has now come nearly completely out of its slot - in the process leaking a decent amount of brake fluid.

I've wrapped the front right brake in a couple of plastic bags and am calling it quits for the night.

Is the fix just as simple as pushing the piston back in, or am I better off replacing the seals?

I am assuming I will need to bleed to brakes after topping up the fluid to replace what leaked out?

I would say that you will need to replace the piston seals - in fact, do a complete caliper overhaul at this point. Yes, you will need to flush and bleed when done, both sides - especially as you are leaving the system open as brake fluid is absolutely hygroscopic and loves absorbing water from the air. That's a bad thing when you heat up the brakes with use.

The topping up and bleeding will all be necessary when your braided lines arrive any way - you'll be draining and replacing just about every drop of fluid.

In the meantime, you could just do as you say - pop the piston back in, clean it all up and attend to the fluid deficit. If the piston doesn't give you any further indication of problems, no fluid weeping etc, you might be able to live with it like that.

When you do as the guys above said, take lots of pictures. It might be of use to others down the road...

These will help: (they are for different bikes but the principle is the same)

If there is no indication of leakage or deterioration of the seals there is no need to rebuild the calipers.

All that needs doing is to drain all but about 6mm of fluid from the reservoir, unbolt the caliper from the fork and hold it in a position where the piston you are reinserting faces up, fully remove and thoroughly clean the errant piston as well as the seals, coat it and the seals with fresh brake fluid, fill the bore the piston works in with fresh brake fluid to just below the lower edge of the seal, and reinstall the piston just far enough to engage not only the dust seal (the outer one) but also the primary seal. This will displace fluid and air into the reservoir, so be ready to keep your bike from getting a bath.

Once the piston is in far enough to insure a positive seal position the caliper such that the banjo fitting is at its highest point and push the piston in slowly until it is fully seated in the bore, then push the other three in fully as well. This will force any remaining air back up the lines into the reservoir.

Using this procedure usually results in a system that needs no further bleeding.

Good luck!

Rob

The above method works.
I was using it 40 years ago and it worked very well.
My buddies instead did the classic bleed, and rebled a few times re our twin disc CB750s that we all had.
I used Rob's method, didn't need to bleed, and went riding.
My buddies would foam and froth while bleeding and rebleeding to get all the air out before being able to go for a ride and even then they often had levers a bit on the mushy side.

Thanks for the post, Rob, as ever.