1990 Audi 100 brake issues

[Peter Elias]

The red "brake" warning light on my 1990 Audi 100 (333,000+ miles) is currently on (not good...) and has been that way for a couple of months, but the brakes work fine (pads/rotors/calipers are all ok, the master cylinder is only about 40,000 miles old, and there is no leakage of brake fluid.

A couple of days ago (when it was about 103 degrees), the brake pedal started going to the floor!   Pumping the pedal, I managed to stop the car and then drive home slowly.

Checked everything under the hood visually and nothing seems amiss (see first sentence above).

The next morning, I had full regular braking power (brake warning light still on though...).  Been driving it just fine for the past 2 days.   A local shop looked at it and couldn't duplicate the problem, only suggesting that the following are replaced:  Brake Accumulator, Brake Accumulator hose and brake pressure switch (all of these in order to get rid of the "brake warning light"..).

Now, remembering back about 2 months, my AC compressor went out, taking the alternator and AC belts with it.  I was left with running the car on battery power only back to the shop.  It was about 20 miles from where the compressor went to the shop and the car drove fine (although it was 105 degrees that day and a bit hot in the car since I didn't want to even run the fan since I needed the battery to power the car...) and as I was close to the shop, the brakes had a similar issue.  The AC was under warranty and fixed for free and when I picked up the car; the brakes worked fine.

For what it's worth, the "low brake pad caution symbol" is on as well (but that is due to somehow the wire coming out of one of the pad got kinked, and is sending this signal; the pads and rotors themselves are new). 

The power steering fluid reservoir is also new and full (since low power steering fluid does actually trigger the "brake warning light" as well)

So, anybody have a clue on what I need to investigate?   The only thing I'm wondering if the excessive heat here in the Dallas area has "something" to do with this phenomena.   I know these models have the convoluted "BAM" configuration.

Will an issue with the BAM cause this?   Does the brake pressure switch affect this (electrically) somehow?   

All I know, is that I ended up with little or no "pressure" applied to the brakes and the next day it was fine.  Kinda scary, but hardly a reason to get rid of the car.  Unfortunately, there aren't too many old-school Audi experts here in Dallas (one of the dealers does have a guy that actually worked on these cars when they were new; but that dealers also charges well in excess of "recommended" prices for parts and $140/hour labor "according to the book" isn't cheap either...).

Thanks in advance for any help!
Peter in Texas