ive just fitted mk3 mondeo callipers and st170 front discs to my puma, and i have not noticed this.

are you braking in a straight line of halfway around a corner lol??

fyi mine has the std drums on the rear and the handbrake is awesome, so i dont think they are an issue.

if braking while turning then with more powerful front brakes the rear end is of course more likely to want to let loose as already said the weight distribution to the front will be greater, thus lightening the rear end and that will have a brake compensating valve fitted and will allow mor fluid though to the rear drums makng them easier to lock (as more weight will be on the front tyres)

all braking should be done in a straight line while the car is balanced