Some things I know about breaks!
The 635 is nearly a 2 tonne car and even the latest model is now 20 years old.. so don’t expect them to be brilliant. They work of course but they’re not as good as modern cars clearly. It’s possible to bolt-on-upgrade E32 750i front brakes and E34 M5 rears, I have heard of a few people having it done. I believe a 750i master cylinder is recommended too if you go this route.
The 635 uses an accumulator system to work the brakes, it’s a bit of a one-off with a few cars of the era opting for it and ultimately dumping the idea in favour of servo-assistance or nothing at all. The accumulator itself (known as the “brake bomb” becuase it looks a bit like an ACME bomb from a cartoon) can fail and it’s very easy to tell when this has happened because you have absolutely no emergency stop capability whatsoever.
If you push the pedal down suddenly and it feels for half a second that nothing is happening the bomb has definitely gone. Having said that, once the initial “fuuuckimgoingtodie!” feeling has gone, the pedal does soften and the brakes will function perfectly, it’s just that initial delay that’s dangerous, so get it changed (local garage did mine for £230). The pedal should be consistent all the way down. Worth mentioning that my car passed it’s MOT with a failed brake bomb.
Make sure that the yellow ABS light comes on when you turn the key in the ignition. It should come on and then go out a when the engine starts. If it stays on this indcates a problem with the ABS system.
Should the ABS light come on and off intermittently when you’re driving or just above a certain speed then it’s almost certainly the sensors on the wheels that have gone. The stantions that flick past the sensor to indicate how fast the wheels are turning tend to rust after a few years and when the computer can’t read them properly or detects an incorrect reading it shuts the system down as a precaution.
You’ll need new wheel bearings on the front since the stantions are built in and you can buy just the stantions for the back since they’re seperate.