mondeo brake problems

Hi all,
I have a 2001 tdi mondeo ghia with a problem with the brakes. Sometimes the brake pedal is soft and goes to the floor and then
without warning the brake pedal goes rock hard and the car feels like is isn't going to stop. When the pedal goes hard the abs activates causes me to skid down the road. It happens more at low reves and when the brakes haven't been used for a while(i.e. motorway driving),or when i'am driving downhill. I've also had a problem with the hand brake, which I've changed but the rear right wheel keeps getting very hot like the brakes are sticking on. I've gone though two lots of rear pads in 7 months but the garage says they can't find anything wrong with the rear brakes. I've been told its possibly the master cylinder or the brake servo pump because its a diesel but no one with give me a firm answer.
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says...

Well the lack of brake pedal can be caused by the brake fliud overheating because the rear right wheel is binding. If you've changed the handbrake cable, the other option is a seized caliper. Rear pads should last years, not months.
A hard pedal is usually a lack of servo assistance. AFAIK, the vacuum pump is on the back of the alternator or powered by the alternator belt so check the alternator belt tension.
If the garages you are going to can't give you a firm answer, try somewhere else. Being diesel makes no difference apart from the vacuum being generated by a pump which is just as easy to check as a petrol car.
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