Lately my rear brakes have been locking up, even after a "rebuild" on the rear calipers. Only after city driving (lots of braking) the pressure seems to build in the rear portion without releasing.

I, on a hunch, decided to try cracking the rear brake line loose up at the master cylinder and..behold, she rolled again.

What could this be? New master cylinder time?

__________________
Vehicles:
2002 SLK 230
Gone but not forgotten:
1983 300D
1981 El Camino 'OILBRNR' - 6.2L diesel
OM617 powered '86 F150
1984 BMW 524td
2001 VW Beetle TDI
1994 Sunurban 4x4 6.5L diesel

The rubber seal in the master cylinder is not clearing the return port for the rear brakes when you come off the brake pedal. I installed the ten dollar master cylinder kit when they were available. Not sold on ebay anymore because of potential liability issues but some suppier out there has them I would think. I would like to grab a couple more of them if I could.

if you dont know when the back brake lines where replaced then replace them. they can swell up on the inside and hold pressure. that holds true for the fronts as well.

__________________
http://i148.photobucket.com/albums/s...titled-1-3.jpg
1981 300d

BoomInTheTrunk - Good advice. But as I said, the vehicle operates normally once I crack the pressure at the master cylinder. Heck, it actually lurched a few inches when I released the pressure. My lines are in fine shape.

barry123400 - Is the only other option, then, a new master cylinder? Or am I better off taking it apart and inspecting for myself? Maybe blasting out the gunk in it and honing the walls?

__________________
Vehicles:
2002 SLK 230
Gone but not forgotten:
1983 300D
1981 El Camino 'OILBRNR' - 6.2L diesel
OM617 powered '86 F150
1984 BMW 524td
2001 VW Beetle TDI
1994 Sunurban 4x4 6.5L diesel

Sounds like your MC is going out. Last time i took my MC apart, it ended in catastrophy. Once you take those rubber seals out, they never will seal correctly again.

__________________
Have you ever noticed that anybody driving slower than you is an idiot, and anyone going faster than you is a maniac?

As long as they would add one additional commandment for you to keep thy religion to thyself.
George Carlin (Wonder where he is now..)

1981 240d (engine donor 1983 240d) recently rebuilt engine hurray! - No more.. fought a tree and the tree won.

pearl black 1983 240d 4speed (Converted!@$$%) atleast the tranny was rebuilt.

Settled then!

Good bad or ugly: Advance autoparts reman master cylinder? Cardone I believe. $50

__________________
Vehicles:
2002 SLK 230
Gone but not forgotten:
1983 300D
1981 El Camino 'OILBRNR' - 6.2L diesel
OM617 powered '86 F150
1984 BMW 524td
2001 VW Beetle TDI
1994 Sunurban 4x4 6.5L diesel

i went to the junkyard and pulled one, it worked fine never leaked. was only 5$ had nothing to loose.

__________________
Have you ever noticed that anybody driving slower than you is an idiot, and anyone going faster than you is a maniac?

As long as they would add one additional commandment for you to keep thy religion to thyself.
George Carlin (Wonder where he is now..)

1981 240d (engine donor 1983 240d) recently rebuilt engine hurray! - No more.. fought a tree and the tree won.

pearl black 1983 240d 4speed (Converted!@$$%) atleast the tranny was rebuilt.

Rear brake hoses are the usual suspect as stated above. They can look great but restrict returning fluid to the master. By you cracking the line at the master, you relieved the restriction. Try cracking open the line at a rear caliper first and if the tire rotates, replace the hoses.

__________________
81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.

It *can't* be the flexis, because the OP can make the brakes release by opening the line at the master cylinder. For pressure to be held so far back up the system, it must be the master cylinder itself.

If it were the flexi hoses twisted up, or failed internally, as sometimes happens, the only place to release the pressure would be at the caliper.