Front pads wearing out very quickly, then discovered that my rear brakes aren't working at all...fluid OK and no apparent leaks, but no pressure. Where do I look for the problem?

Start with trying to bleed the rear brakes.
Could be a bad master cylinder also.

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77 W200 440/727/NP203/4.10 All Stock

Check the proportining valve on the left frame rail to see if it is working properly.

try bleeding them, check proportioning valve as stated above, and make sure they are adjusted. How does the pedal feel? Hard? Lots of travel?

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Does anybody else have crappy rear brakes? If I put mine on jack stands, turn it up to about 70mph and hit the brakes, my rear wheels dont stop. I think its the proportioning valve, but I dont want to find out for 130 bucks. When driving, if I hit my brakes, th epetal goes almost all the way to the floor, it stops fine, but something is wrong. Everything is new except the proportioning valve. Maybe this will help you too dgmartin_054