Brakes. Again

I posted just recently about my brake pedal becoming hard and the brakes binding after a short time. I have also been reading about the'bomb' Now Because I don't have a book I don't know where or what this is. I do have
a canister maybe 10"long to the left of my master cylinder but I assume that's the power steering reservoir.
Now I trust the experience of the people who replied to my last post but I need to add some more detail just to convince myself that it's the master cylinder at fault.
Ok here goes.
1991 5cy 10 v ABS
When starting out the brakes are good very progressive and the ABS light NOW comes on at 15mph. After maybe 4 5 miles the power of the car drops off as it struggles to fight the brake bind.
As previously suggested I tried cooling the master cylinder with cold water but this did not work. The brakes remain solid for some time at least until something has either cooled down or the pressure has dropped naturally.
In addition to this if I push the brake hard whilst not moving and cold the pedal goes to the floor with what seems a series of pulses...
I cleaned the calipers and new rotors and pads were fitted (all round)just before I got the car. Is there anything else which could cause these symptoms?
I guess I am looking for confirmation of the MC problem. This is my first Audi in the US having lived in England and I have no experience of brakes without Vacuum servo
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ian wrote:

The 'bomb' is called that for two reasons: 1) it contains pressurized nitrogen at 2,500 psi and you don't even want to puncture it. 2) it is shaped like one of the cartoon bombs with a fuse. Round, black and about the size of a grenade.
It is fairly easy to change and is located under the car just in back of the air conditioner compressor.
This doesn't sound like your problem. It only is used to assist braking if the engine kills for some reason. With the hydraulic pump working the brake assist is fine even if the bomb has gone bad. The symptom of a bad bomb is the brake indicator light flashing briefly under hard braking when the system sees low pressure from the sensor just behind the brake master cylinder.

Probably an ABS sensor at one of the wheels is out of adjustment. Have you changed an axle lately and installed a new one with the wrong number of teeth on the ABS ring? different years had different numbers of teeth.
After maybe 4 5 miles the power of the car drops off as

Still sounds like MC ...

Leaking MC O rings and the pulses is from the ABS.

Are the sliding bolts for the calipers free so that they can center and not bind on inside or outside?

Still convinced that the MC is your problem.
The only other possibility is the brake proportioning controller by drivers side rear wheel could bind but that usually results in the rear brakes not operating rather than binding. This device reduces the pressure to the rear brakes relative to the fronts so that the rear won't try to beat the front of the car while breaking in slippery condition (spin-out).
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Ok I feel happier now. Your explainations makes sense to me.
I am very used to the caliper set up on the Audi it's a dream to change pads so I am confident that they are well cleaned and operational.
thanks again for your insight.
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New master cylinder cured the problem. Many thanks and seasons greetings
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Thanks for letting us know the result.
Glad you got it solved.
Tony
ian wrote:

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