Got a brake question id like to run past you guys if you dont mind. Im pulling my hair out trying to figure this out. I have both rear brakes locking up on me and the problem seems to be intermittent. I rebuilt and inspected all 4 corners including disassembling the calipers for cleaning, fluid has been flushed no less than 3 times, master cylinder functions properly. No ABS codes either, actuator seems to be functioning correctly and passes fluid. Still have the issue. Any idea if they are still using residual check valves in disc brake systems or is that a thing of the past? Brake issues are usually pretty cut and dry but this one is refusing to be vanquished. The fluid was exceedingly nasty, it was actually sticky to the touch. Just bought this car and wasn't able to get around to flushing it before a problem arose. Any experience with the symptoms of petroleum in a brake system? That is the only thing i can come up with to create sticky brake fluid. Any thoughts?

1995 BMW 540i

Generally, petroleum will just eat all the seals and you'll have brake fluid leaking out of every seal in the system.

Not sure if your car would have a residual check valve, some BMWs did in the ABS pump. However, check your rear flex hoses- they can swell up on the inside and act just like check valves. This happened on my '89 325is and a classic Beetle I owned.

This may sound dumb, but it isn't just on dewy or after rainy nights is it?

My first inkling would be that the old fluid was so gummy that the rear pistons stick. Are the brakes grabbing and locking at the slightest touch of the pedal, do they lock on their own without pedal pressure, or are they just kinda grabby and fail to release properly?

Just failing to release correctly. Not holding full brake pressure but holding enough to slow the car down and heat it up. It IS both rear brakes as well, not just one side. There are no rubber brake lines in the system that could effect both brakes. Got a master cylinder on the way and a set of brake hoses for the off chance both flexible lines have failed. Also ordered a set of rear calipers in the event there is some dimensional issues with the pistons. Found calipers for $25 apiece so not a big expenditure to completely eliminate the rear end. Im thinking that the residual pressure check valve is sticking. I was able to confirm that they are built into the master cylinder. Everything in the system will be eliminated except for the ABS actuator which is $$$$$. Parts are cheap enough that i dont mind diagnosis by attrition lol. With how nasty that fluid was it is not a bad idea to replace as many rubber components as i can. Ive seen fluid in cars that have sat for 40 years that was more useable than this crap.

And no rainy or dewy nights around here, very dry climate in CO right now.

Finally got it figured out. The problem was in the master cylinder. What a strange master cylinder issue to have, no change in pedal feel at all and it appeared to be working. Good to have it back on the road! Now to go find some ricers to put in their place