* Every now and then I see a car with brake hydraulic problems which stem from something or other that has swollen the rubber components in the master cylinder. The fluid surface in the reservoir looks like the surface of a vinaigrette salad dressing, with the oil globules floating on the surface, and there's usually plenty of sludge on the floor of the reservoir.
* Has someone mixed silicone-based DOT5 with the DOT4 to get this mess? Or is it an age issue or a moisture issue?
* I normally use ATE SL DOT4 fluid in all my brake work, and as far as I know DOT3 and DOT4 are compatible (if they happen to end up in the same reservoir).

I believe DOT 3 & 4 can be used together, although not recommended, with the difference being that 3 absorbs more moisture than 4. So I suppose 5 absorbs even less. I use 4, just as you do. In any case, the master cylinder sounds like a system that has never been bled or cleaned.

* 280EZRider, but I've worked on dozens of really skunky brake hydraulic systems, which have never been fluid-changed, and there's not been any swollen rubber. A certain amount of sludge but no deformed rubber.
* During a fluid change/flush, the master cylinder piston travels the full length of the cylinder and, if there's rubber damage, the pistons won't return properly. Of course this necessitates a new master cylinder and a good flush (which is totally warranted given the absence of maintenance) but I still wonder what is working on the rubber, since not all the unmaintained hydraulic systems fail for this reason (whatever it is).

Swollen seals are caused by petro product contamination.
These include mineral spirits, WD40, degreasers used in cleaning, and I have even seen Power Steering fluids mistakenly used to top off brake systems.
Contamination can be seen by draining off some fluid into a clear jar and see what settles/layers over-night.

Yeppers, power steering fluid or even ATF.

Thank you Jiffy Lube...

__________________
Palangi

2004 C240 Wagon 203.261 Baby Benz
2008 ML320 CDI Highway Cruiser
2006 Toyota Prius, Saving the Planet @ 48 mpg
2000 F-150, Destroying the Planet @ 20 mpg



TRUMP .......... WHITEHOUSE
HILLARY .........JAILHOUSE
BERNIE .......... NUTHOUSE
0BAMA .......... OUTHOUSE