I read your problem as the car steers on application of the foorbrake and then brakes in a straight line. Steering straightens when footbrake released?
My geustimation is either shot whishbones, steering idler or wheelbearings.
Start with the latter. Jack up one front corner and feel for free wheel play by holding wheel at 3/9 position and rocking wheel in/outas though trying to manually steer the car, if you follow. Repeat holding at the 12/6 position. Remember what both feel like. Have someone depress and hold down the footbrake and redo the 3/9 and 12/6 test. If the perceived movement is drastically reduced, its the wheelbearing thats duff. If the movement is essentially the same, its a steering/suspension fault and requires further investigation.
With both front wheels free of the ground (on axle stands, please, not bricks or trolly jacks....) have an assistant move the steering wheel from about the 10 oclock to the 2 oclock position and back repeatedly whilst you view and feel all the steering linkage points for free play. If any found, may require a qualified cleverv person (MOT Tester?) to say whats allowed and whats excessive.
Whilst car up in the air, you could carefully use a pry bar or large screwdriver to see if there is any free play in the lower suspension wishbones but, to be truthfull, this is a more specialised/experience based assesment and not something that a basic roadside DIY'er may be able to do properly.....
