I have a 1991 Honda Civic wagon that loses braking power on hot days. I have it working for the initial press, but as I hold down the brakes I slowly lose braking. I have to repeatedly press at red lights, and after a long day of driving my leg gets tired (ha-ha). Still on a cool day the brakes work without problems. A friend of mine said it might be the brake master cylinder, but I wanted to make sure.
I had brakes issues with my 1987 Toyota Land Cruiser since 2 years, I've had the same symptoms has you.
Since then I've change all brakes (rear/front), replace some flexible brake hose that were soften due to age and anti-rust treatment with stainless steel hose(might be overkill). Then I bled all four brakes to remove air in the lines which give me a firmer pedal.
Now I have brake but I have to depress my pedal way down to start braking. I know that my master cylinder is faulty because I had bleed it in place and my pedal got less travel to get the same result.
So, to answer your issue:
Replacing any component of the braking system you'll need to bleed the system according to the manufacturer specifications.
If after bleeding the brakes and checking the mastercylinder check the replaced brake line. Did you install it in the exact same position as the original? If it passes to close to an exhaust component the heat from the exhaust may be boiling the brakefluid. The boiling fluid would contain a bubble of air that is compressible resulting in low pedal pressure but only while the engine and the exhaust is warm.