Mercedes Forum - Mercedes Benz Enthusiast Forums - Riddle of two-tone brake fluid

I have a 1988 W124 230TE which I have owned since 1995. I bought it with 42000 miles on the clock, and it now has just over 314k. Now on the second cam, it runs almost exclusively on LPG, and Mobil 1. I have carried out all servicing and maintenance since I had the car, and it has gone through every MOT with flying colours. Although the body is now needing a good amount of cosmetic TLC she's still a very presentable car. Interior is almost unmarked black cloth - amazing after carting a family with 4 kids about for the last ten + years!

However, Tuesday last week, driving along the motorway after a spell in city traffic, the brake pedal went very hard and 'high'. It usually bites about 1" down from the top, but now was hard as nails. No loss of braking efficiency, just a much higher than normal pedal bite point. Stopped at services and did the usual servo test; engine off, pump pedal till hard, press on pedal and start engine. Pedal went down about an inch or so..... as you'd expect.

It was just short of two years ago since I changed the brake fluid, so that was the first suspect. Got the car off the ground yesterday, blew all old fluid out of the system with clean dry air. No residue in the bottom of the reservoir, BUT the fluid has a distinctly unusual appearance. Not sure how to post a picture yet, so please e-mail for one! (or tell me how to post one!)The large container holds the fluid from the front brakes, the smaller container from the rear.

All I have ever put in the system has been clean, new, fresh DOT4!

Baffled, I refilled the system with clean new DOT4 and tried the car. Same symptom, BUT further investigation revealed that if you get the car to go at a reasonable speed for a while the brake pedal is 'normal', but as soon as you hit traffic and the under-the-bonnet temperature rises, the pedal goes hard.[IMG][/IMG]

So I put the smaller container into a microwave with a viewing window, put it on the lowest setting and gradually heated it up...... sort of. The brown ('brake fluid') liquid heated up quite easily. The green showed very little temperature rise at all. Both behave as 'oils' rather than a water-based liquid, and both 'cling' to the side of the containers. A vigorous shake, and although they mix initially, they separate out in less than 30 seconds.

Anyone got any clues?

No other mechanic has been anywhere near the brakes; in fact the only mechanic to be near hte car has been an MOT tester who I trust (one of a few in number!) but it's the weekend, and he's entitled to some time off!

ANY suggestions as to cause and solution (sorry, no pun intended) would be greatly appreciated.