Sudden brake failure

1990 740 b230ft. Two months ago I replaced my front brakes and flushed the whole system. It was a success, the car was braking fantastically. The car sat for two months, today I went to go for a drive with new suspension and the pedal went straight to the floor. Was a little low on fluid, topped it off, made no difference. No visible leaks. Pedal builds up zero pressure at all.

Sounds like a bad master cylinder. If you're losing fluid and yet see no visible leaks, try loosening the master cylinder off the brake booster and see if there's any fluid making its way past the seal into the booster; that's a clear indicator of a bad master cylinder.

If you just worked on your suspension, though, take a good look at any brake components you've handled.

I did this, didn't see any fluid leaking out of the master cylinder. Still haven't found any leaks and the fluid level in the reservoir stays the same. Could something with the abs be causing this?

daniels740 said:

Sounds like a bad master cylinder. If you're losing fluid and yet see no visible leaks, try loosening the master cylinder off the brake booster and see if there's any fluid making its way past the seal into the booster; that's a clear indicator of a bad master cylinder.

If you just worked on your suspension, though, take a good look at any brake components you've handled.

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Nice puzzler. Hope not to find out what you flushed the hydraulics with turned out to be something other than the dot4 you thought it was, which took some time to destroy seals. Assuming replaced the brakes means new calipers I think you'll need to plug the master ports to decide which end to work on. Losing fluid into the booster isn't the only way a master can fail on both circuits. And you don't believe you are losing any.

When you flushed the brake system, did you use the pedal pump method? If you did, did you put a block under the pedal to prevent the brake pedal from traveling past its normal range of motion? If you did the pedal pump and didn't block the pedal, you have likely damaged the piston seals on the MC on the wear ridge that develops after years of use. The failure may not appear immediately; but, after a couple of days of use (been there, done that, got the Tee shirt). When the piston seals fail the fluid leaks back up into the reservoir so there is no visible sign of leakage.

To avoid this problem, all brake system flushes I now do are with a pressure bleeder. If you used a pressure bleeder or vacuum bleeder or you blocked the pedal I don't know what the source of the problem is.

I bought a Motive pressure bleeder and used the same dot 4 fluid on my 240 when I bled those brakes ~10,000 miles ago and the brakes are working great. I think its possible I’ve over extended the master cylinder and worn it out quicker than it should have. Before I replaced the brakes one of the lines failed and I remember slamming the pedal all the way to the floor at least once. I did replace the front calipers, I didn’t touch the back brakes at all only bled the calipers. Will perform more diagnoses tomorrow when I have time, thanks for the tips.

May I suggest that while you are rebuilding the entire braking system that you add the replacement of the six rubber hoses at the front wheels and at the rear connected to the pressure regulators.

Those hoses are probably 32 years old now.