Inconsistent braking - need help figuring out the problem

To preface: I'm almost positive I'll get the car checked with a mechanic either way, but I'd like to get a better idea of what might be going on before that.

The car (a 2014 Renault Duster) has front disks and rear drums; this summer the rear cylinders started leaking, so I ended up replacing both cylinders along with shoes + changing the brake fluid. The guys at the shop swear they inspected every inch of the car, including the master cylinder, and found no other issues with the braking system. I took the car back to the shop for an annual checkup in November, and they once again said they weren't seeing any problems; checked the front brake pads and said there were 10 mm out of 12 mm remaining. 

May or may not be relevant: we've had lots of snowfall this winter, and now that the temperatures are rising above freezing levels, all this snow is melting. Some roads are relatively dry, some have puddles, some are flooded.

So, the problem: When I hit the wet roads for the first time, it was fine for about 10-15 mins, and then I noticed that the car wasn't braking quite as it should - took more time to slow down and I wasn't feeling this slight resistance of the pedal when braking. I took the car to a quieter place, tried braking hard on low speeds a few times, and sometimes it would slow down relatively alright (although still not quite as it normally should), sometimes it would brake veeeery lazily, barely slowing down, but I couldn't really figure out a pattern to it, so I gave up and parked the car. Went back to it in a few hours to move it to a flatter surface, tried braking hard again and almost hit my nose on the steering wheel lol, it stopped instantly, but then in a few minutes started braking more lazily again. So the problem comes and goes, and it looks like the car brakes as it should after being stationary and after a while starts losing its brakes, but not in any consistent manner. 

What could be going on? Initially I thought that maybe some water got inside the drums, but then I suppose it should've dried out and returned to normal while I was braking? (I also have a feeling that sometimes brakes were getting worse on a relatively dry stretch of the road, but I'm not perfectly sure.) I only have a bit over a year of driving experience, and it's my first time driving this car in such conditions, so I just don't know how it reacts to floods and deep puddles. If that's not the water, however, I'm thinking it could be a leak (but then it would've been more consistent, no? and then I checked the brake fluid levels a few times, but I'm not seeing any changes, although if it's just a very minor leak, I wouldn't notice it); or some kind of ABS failure; or maybe the master cylinder failing. Anything else I'm not seeing?

Questions:

Is there any way for me to confirm/rule out issues with ABS or the master cylinder? Any feedback from the pedal that might be pointing towards one or the other, for example?

What else might be causing this? Anything else I could try in a quiet place before taking the car to the shop?

How long should a proper, thorough inspection of the brakes take? 1h? 3h? What should they look at? 

Bonus question: some people seem to be able to tell if the problem is with the rear brakes or the front brakes based on the car's behavior: is that possible? what should I pay attention to?