Your brake master cylinder is actually two cylinders in one, with a common fluid reservoir. One cylinder applies front brakes and the other applies rear, both when you step on the brake.
Each cylinder has an inlet port to get its fluid from the reservoir. It has an outlet that goes to its respective front or rear brakes. It ALSO has a little VENT port, to make sure any fluid returning from the brake, when you release it, gets back into the reservoir and does NOT leave that brake applied.
It's only a pinhole, and I've had one "heal up" with corrosion. It would slow the car down on the open road. One time it stopped the car. When I got out, both front wheels were like walking past BBQ grills.
Replacing hoses is a good idea, and it's good you did it. But in this case, I agree with others. I agree that it's a common problem source, and I've actually had it be a plugged port in the Master Cylinder.
I fixed it after two dis-assemblies. First time I missed it. Second time I saw that a little "spot" in one cylinder chamber matched a little hole in the other. A wire wouldn't poke through. I used a small "number drill" and ran a hone over the bore surface. That was to make sure I hadn't created a burr to nick the sealing surface of the rubber piston cup.
Found you a diagram to show what I'm talking about.
The Pink and the Green Chambers/Circuits have two passages each, coming down from the Reservoir. Notice how the RIGHT one in each pair is drawn coming to a point at the bottom. That represents the pinhole that I think is blocked in this RV's master cylinder. If that hole is NOT open, pressure can remain in that brake circuit. The hotter the brake gets, the more the fluid expands, the more unintended apply pressure goes to the brake calipers. BOTH of them.
