1991 Toyota Corolla Help please! Possible vaccum leak?

Engine Performance problem
1991 Toyota Corolla 4 cyl Front Wheel Drive Manual

Hi
I would appreciate any advice on this problem. When starting the car in the morning, the engine will idle fine while its cold. Once its warmed up it will idle rough and even more badly every time I step on the brake.
Furthermore, this car is losing water, like a litre everyday but I cant see where the problem is. I checked the oil for "milkshake" color, but the oil is clean. The oil cap also has no indication of any moisture. I hear a slight hissing sound coming from the intake manifold side of the engine but see no water on the ground and no signs of vapor. However a few times, my windscreen has misted up quite badly for no reason. The heater core does not appear to be leaking, I checked the passenger side for dampness. I am at a complete loss as to what is causing this problem. It has a new radiator, radiator cap and thermostat. Do I need to replace the hoses on the car as this has never been replaced in the lifetime of the car?

Thank you in advance for your advice and suggestions.