[ChrisZ]

Well this was interesting. Last time I drove the 86.5 928S I noticed the brakes were a bit soft. Odd, so noted it and parked it and with a lot of pressure I felt the pedal sinking. Except it's 30 degrees outside so the fluid (1 year old) is not bad.

Fired it up this afternoon and took it around the block. Brakes went from good to gone in seconds, fortunately I was driving slow with my hand on the hand brake, but the pedal went to the floor and there was nothing slowing it down but the emergency brake. Turned around, drove it back home slowly, had to use the E-brake going down my driveway to stop it.

Not good at all. No brake warning light, either it's burned out or the switches are bad but my guess is it's the master cylinder. Checked the fluid reservoir, up to normal levels, no leaks under the car.

Car is offline till spring (pulling the battery so no one drives it by mistake); I have the 944S anyway so am good to go.

Any thoughts on replacing the master cylinder? Do the fenders need to come off? Engine come out? :-) Any issues bleeding the ABS system?

Thanks! Wild day...
Chris

[worf]

The brake master cylinder is a straightforward(*) R&R. It's the clutch master that is "fun."

Problem is that new BMCs are either NLA or cost as much as college tuition (I forget which but the end result is the same.)

Mark offers rebuilt units. Make sure that the end of the booster's pushrod is shaped *identically* to the inverse shape of the rod receiver in the replacement BMC. There's a "smooth end " push rod and a "round with a tiny locating pin" push rod. And BMCs with an accommodation for the tiny pin and BMCs without. If you've got a pin on your rod and no accommodation, the pin has to go bye-bye.

You don't need to worry about the ABS.

* As straightforward as any R&R for a part that hasn't been touched in 40 years. You may need an inductive heater for the hardlines.

[ChrisZ]

Ok, well after 5 months of going out weekly, pouring penetrating oil on the bolts and brake line to cylinder interface everything came off with pretty much ease. Helps to have 1/4 inch tools to get into the tight places and Snap-on won't snap :-)

Anyway cylinder is off, it has a round interface to the brake booster and the pin that presses on it is round. Time to get a new one, then the car can get back on the road. This one looks... Sad.






Just takes time to do....

[Sazerac]

Congratulations on that approach that allowed for easy removal!

Stan's frequent commentaries OTOS about brake master cylinders has largely created my set of prejudices about them, which have been subsequently confirmed by experiences in my old 928s and SAABs:
1.) Brake master cylinders are a wear item
2.) Strange brake behaviours are usually an early sign of BMC failure

I have also then followed Hacker's advice of rebuilding all sorts of hydraulics stuff at Whitepost (www.whitepost.com). So far so good. The only thing I don't like about them is the lousy black paint they use to repaint the cylinders. Your cylinder is quite sad looking though, so you may want to try rebuilding a nicer one.

[ChrisZ]

Whelp home from SITM and going to ask Jeannie to get me a new cylinder. The old one looks like the seals just failed.

So I took a look at the reservoir. It is of course filthy on the outside, however washing it out is causing all sorts of rust and gunk and whatnot to come out. It's filthy on the inside as well, which is.... odd.

I may just replace that as well; any thoughts on cleaning it? Thought about just dropping the whole thing in carb dip, but worried that would wreck the plastic or whatever it is made of....

Not too good here, not too good.

[worf]

So I took a look at the reservoir. It is of course filthy on the outside, however washing it out is causing all sorts of rust and gunk and whatnot to come out. It's filthy on the inside as well, which is.... odd.

I may just replace that as well; any thoughts on cleaning it? Thought about just dropping the whole thing in carb dip, but worried that would wreck the plastic or whatever it is made of....

Use a 'soap' not anything acidic or caustic. I prefer D-Limonene-based cleaning products for 99% of 928 cleaning tasks.

Any non-foaming "citrus power" product will do. Or even a foamy one. I use P21S Citrus Total Car Wash concentrate diluted 1:1 or 2:1 with distilled water.

Get some plastic push nuts to seal the outlets. Fill the reservoir. Shake. Let it Sit. Shake. Tip it on its side. Let it sit. Shake. Sit...

You get it. Once the water/cleaning agent is black. Dump it. Lather rinse repeat until the res is clean.

Rinse several times with distilled water. Do a final rinse with alcohol.

There are baffles in the res, so it's a process to clean and *thoroughly* rinse it.

[ChrisZ]

Ok, but this is not totally "unusual" I assume

[worf]

Ok, but this is not totally "unusual" I assume

The black shit in the reservoir is a mixture of iron and rust.

The reason it's in your reservoir is because the brake fluid in your 928 wasn't changed every two years.

Probably, before you bought it, it sat or ran around for 10 years with no brake fluid flush.