We bought a used 2013 Hyundai Elantra GT (hatchback) 6MT earlier this year. It's my son's DD.

He told me that the brake warning light was illuminating periodically while driving. I just checked this morning and the brake fluid reservoir is just above Minimum fill.

Reaching down to the MC, it's dry. But the problem's been getting worse (at least, the caution light has been illuminating more frequently) so there must be a leak somewhere. The reservoir is quite large -- at least 2x the size of the 914's.

We already have a service appointment in ~3 weeks for other things. Are we good to add some of the correct brake fluid to the reservoir until then?

Any guesses where to look for the leak?

Inspect whole brake system including all hard lines.

Something isn’t right. If you can’t find fluid anywhere on brakes or external lines, it’s leaking from the master cylinder into the vacuum booster canister.

I wouldn’t be driving around with a leaking brake system.

May seem like a minor leak but if you need to do an emergency stop and try to hit maximum pressure you may find out that it will blow out pressure completely when you need it most. Then you’ll only have 1/2 the brake system working on a split system (front / rear or diagonally split).

The light is there warning you for a reason.

If this was the first time you corrected the brake fluid level- no worries!
People don't check that now-a-days nearly enough and it might have just been oversight.

I run an automotive shop and it's common to have this happen.

You have to fill it a second time in a 1-year period I'd DEADLINE the car.

No further use until source of the leak is found.

The ONLY thing that is important on our cars is the rubber stays on the road and the brakes work. If it starts and moves under its own power, it just means you don't have to push it around the shop.

Look for the wet spot in the system.

It is possible to have it leak into the booster but commonly you will see destroyed paint on the booster housing just under where the master mounts.

Fill the reservoir take a black sharpie and scribble over the warning light and send it down the road. It’s a thirteen and already bad luck

It's a measured system. When the fluid is low that means the pads are near worn out.

DON'T TOP OFF. It will all just come out and eat the paint when new pads are installed.

At 10 years it's ready for a fluid flush, so when you get new pads & rotors, bleed the system to replace the fluid.

For some reason I read the OP as it already having had fluid added a couple times but re-reading, it doesn’t say that. I read leak and assumed that it was leaking because OP said leak.

It is conceivable that pad wear has simply dropped fluid level and the light coming on more frequently is occurring due to the low fluid level being right around the on/off switch threshold and sloshing fluid in the reservoir. Especially if vehicle has aftermarket pads with backing plates that are thinner than OEM and pads are worn well past wear limit.

Yes, a combination of pad AND rotor wear will bring down the fluid level. Check to see if any of the brakes are excessively hot indicating a seized caliper. That will cause excessive wear.
Photos have been posted of a brake rotor worn down to the internal vanes on one side. I had a 914/4 rear caliper seized and the caliper piston was wearing on the rotor; having worn away the backing plate on the pad. Amazing that in both cases the pistons did not pop out of their bores.

This is where people (and shops) make a huge mistake...

I don't EVER push the fluid backwards into the master cylinder. The fluid in the calipers is the stuff that's the most damaged, this fluid should be replaced.

Pushing old fluid in reverse through an ABS unit could screw that up as well.

The first thing I do when changing pads/rotors is to break the bleeders loose first and lightly snug them back. I do this before breaking the calipers loose.

After I have the caliper un-bolted, I'll stick a vacuum hose on the bleeder into a pan, open it, and push the caliper pistons back by hand.

You can really tell if you have a problem caliper using this method. You are not pushing cooked fluid back into anything. When the job is done, you do have to bleed the brakes a little bit but if you keep the bleeder high it's not bad at all.

Commonly I sell a brake fluid flush at the same time. I'll flush the fluid first, before the brake job, and end up with 99.5% of the fluid changed this way.

Pushing the old fluid back into the brake system will get you FIRED with me...