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Brakes soft, now brakes are locking up! 700 1985

posted by  sdodd on  Tue Jun 30 12:10 CST 2009
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last visit: Sat Jan 23 00:38 CST 2010
[RELATED]

Hello,

I have an '85 745 with a front jumbo brake upgrade. Over the last month I've had a "weak" diagonal (LF, RR) and now the same diagonal was stuck this evening.

About 3 months ago I had the rear calipers off while doing a rear axle swap. I accidentally pressed the brake pedal down without a rotor between the rear pads. Both calipers pressed tightly together. I pulled them back. Upon getting the rotors back on, my RR caliper would not compress and the brake light was on. (brakes worked fine before this) I removed the caliper and confirmed the piston and dust boot were functioning fine with no rust. I thought that the deep compression of the master cylinder screwed it up so I replaced it. Still no luck, so I rebuilt the distribution block/brake light switch. Things worked OK for a while.

After about a month the LF/RR circuit started loosing brake power/pressure. It was intermittent at first (solid pedal, then soft at seemingly random times) but started getting worse. Then, I lost all power in that circuit and the brake light went on again. (rotors started to rust!) As my rebuild of the distribution block was suspect I purchased a new unit. Upon installation I still had no brake power in the LF/RR. I did notice that when I bled the brakes the LF/RR flow was very low compared to the RF/LR. I use a Motive pressure bleeder under 15psi max.

So, I got a new master cylinder to rule out an internal leak. In addition I blew out/cleaned the hard lines traveling from the MC to the distribution block to confirm there was no blockage. Pressure bleeding was good with equal flow in all calipers. Solid pedal feel with no brake light.

Two days later (today) the LF/RR calipers were both dragging enough to cause some shudder while driving. Hub temps on LF/RR were much hotter than RF/LR.

So, new MC, new distribution block. Calipers are both good and slide pins (front) are functioning correctly. I have IPD SS brake lines on all 4 corners. They are ~7 years old with ~30k miles. Only other thing to add is that this is a V8, so there is an exhaust manifold right next to the MC and distribution block. Though, In 7 years of driving I've never seen an issue. Brake fluid is Valvoline Synthetic Dot 3/4.

Any thoughts??? I'm going insane with this!

thanks,
simon





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