why is my 97 540i bmw rear brakes locking up? I replaced rotors,cal.lines,mcyl,?

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a few weeks ago I experienced a rear brakes total lock up after trudging it home it was flaming hot and extinguished fire with garden hose. I know fundamentals of engines and drive train functions but not ABS friendly yet so after three more brake lock ups and a grip of Christmas present money to by my family gifts spent for parts its waiting for a solution and parts I cant afford and top it off with a water pump replacement with a idler pulley and belts and thermostat housing and censor and 4 hundred more bucks Im in a slump and a few words of advise from different sources some reasonable and some not. Seems like my professionally inclined sources are the least interested with the potential mother load and labor they would make off of me and with that said why not tell me up front its more than they can handle and find a bmw specialist. Truth is its a car with more problems than I have money. I like the car very much so I need to find out the right thing to do. The check engine lights on and at this point the brakes are not locked up. I have not tried to do anything since the last lockup . They must have cooled off and returned the rotor pistons so its drivable but Im afraid to go through it again. Do you recommend an ABS MODULE replacement or rebuild or what else can it be.? I also replaced The master cylinder . no leeks of fluid or no air in lines .I used the proper brake fluids recommended for that specific model and make. Its got 184.xxx mls. on it and runs cool and well all functional. Thanks for reading my little problem and please respond with a report on all posiblities.