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Discussion Starter #1
ok searched other posts and found a few saying thier brakes where locking up while driving like mine are but mine the drivers side and passenger wheel r the only two locking up when driving or pumping pedal. i let it sit for a hour and its fine? i also can use a 10mm wrench and open bleader on either caliper front or rear and it relieves the pressure from both the driver front and rear passengerand its good to go for about another 3 miles?? so i replaced the master cylinder took out the abs fuse..still doing it. it just started doing it after owning the car for a few years. they dont just lock up it seems as its building pressure slowely and then it gets so tight i cant move the car. any help would be awesome. thanks
 

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I read a thread a few weeks ago about someones car doing this. His turned out to be his brake lines at each caliper being almost completely plugged from the brake fluid never being flushed and the inside of the rubber lines deteriorating. The problem was the high pressure from the master cylinder was enough to force past the blockage but then wouldnt work it way back out.

Pull your brake lines from the calipers that are locking up and inspect them, see what you find. Depending on mileage on the car I would recommend replacing all your brake lines.
 

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Discussion Starter #3
yea i checked the brake lines they r free i have do full brake flushes every year. my fluid looks as if its band new. what i did find tho is that i can relieve the pressure at the master cylinder on the front brake line and it releases both the driver fron and rear passenger brakes. the other two work just fine lol?!? anyways i ordered another master cylinder thinking that it has to be that? i put on a used one on new to me but used. it will be here in the morning so hopefully this solves my problem. its crazy that the brake line on the side of the master cylinder runs the passenger front and rear driver brakes and the front brake line runs the driver front and rear passenger brakes but thats the way it works on my car lol!! i also took ront brake line off of master cylinder and compressed both front and rear calipers and the fluid came out in a steady stream. i then put the front brake line back on the master cylinder and beld brakes then pumped up the brakes and they stuck again so it has to be that stupid master cylinder?? dont know what else it could be
 

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If the piston in the MC cannot return all the way out the compensation port stays closed. Could you press you caliper pistons in with them hooked up?

It may be an ABS pump issue as well, but checking the MC is a lot cheaper!

-SP
 

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Discussion Starter #5
If the piston in the MC cannot return all the way out the compensation port stays closed. Could you press you caliper pistons in with them hooked up?

It may be an ABS pump issue as well, but checking the MC is a lot cheaper!

-SP
nope couldn't push calipers in while hooked up to MC. So I have a new MC on order should b here in the morning.
 

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I think you're missing Speedy's point. It's not necessarily the MC that's bad. Something could be keeping the piston from returning all the way. Get in under the dash and make sure the pedal assembly is working correctly and that nothing is caught or binding (drooping wires?). If that looks good than be sure to check the pushrod adjustment between the booster and the MC when putting the new one on. If the rod is out too far it could be keeping the piston from returning all the way to the compensation port.
its crazy that the brake line on the side of the master cylinder runs the passenger front and rear driver brakes and the front brake line runs the driver front and rear passenger brakes but thats the way it works on my car lol!!
That's the way it works on most newer cars. And when I say "newer" I mean since the 70's.... ;)
 

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Do they brake lines, they're cheap. The lines will collapse in on themselves. I've never seen them actually lock themselves up but before you go spending a ton of time and money, try the cheapest fixes first.
 

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Hey, anymore going on with this? This is starting to happen to one of my 97's. Drives along seemingly OK, then loses power, gets hot and groans to a stop. Cannot push it for about 30 minutes, then all of a sudden it breaks loose. Easy to suspect master cylinder, but I suspect ABS???
Any help?
Thanks!
 

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Just an update to my dilemma. Found master cylinder to have a layer of crud on the bottom surface of the the reservoir and the fluid dark in color. Removed and inspected mater cylinder and found that the piston(s) would stick just about the port and not returning all the way back, at times. Before removing the MC, I had checked the push rod adjustment and found it to be within spec. After weighing the old fluid, I brought it up to 206 degrees F, the boiling point of H2O at 3300 ft and allowed it to remain there for 30 min. As I calculate this, I lost approx 7% through evaporation. Since I do not know the specific gravity of the old fluid I am not sure if I got all the water out, however this seems like a lot to me.
I did R&R the MC and installed new fluid and all is well now. I have to say that after 30+ years of turning wrenches this is the first time I have seen a vehicle come to a full (service-brake) stop WITHOUT ever touching the brakes. I have encountered a brake lock-up after applying the brakes many times, but not like this. That is why I was trying to find out how much residual moisture was in the fluid.
I do believe that the amount of moisture in the system allowed the fluid to boil at such a low temperature that it would expand enough, given the inability of it to return to the reservoir do to blockage, to cause the brakes to lock after heating and then release after cooling.
Hope this information might be usefulness to someone.
 

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Moisture does not cause this symptom. Moisture causes a low to no pedal when the moisture turns to vapor when hot.

HOWEVER the moisture does cause some terrible corrosion problems in the system. The fact that your piston didnt return all the way was the real cause of your brake lockup.

I had not encountered problems like these in my years of wrenching either until the last few years. I think new formulas for fluid and rubber brake parts have combined to cause all new problems for us.

Keeping fresh fluid in the system is the best bet for reliability.

-SP
 

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I totally agree seedy25. My problem was the MC and dirty fluid. I guess what I was trying to get at was the H2O is lowering the boiling point of the fluid. The MC was not allowing the fluid to return, and in affect "locking" the fluid in place. However when the fluid was heated by the friction of the now "stuck" brake pads, it is, I am assuming, expanding the fluid at a faster rate than just fluid alone. Hope that is making sense.
Anyway, thanks for the input.
 

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When the system heats up the fluid expands. There is a small hole in the MC called a compensation port. This port is the one that squirts brake fluid all over your car if you dont put the cap on when you are brake bleeding. :) If the hole is plugged or covered no fluid can flow back into the MC and the brakes lock up.

-SP
 

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yea i checked the brake lines they r free i have do full brake flushes every year. my fluid looks as if its band new. what i did find tho is that i can relieve the pressure at the master cylinder on the front brake line and it releases both the driver fron and rear passenger brakes. the other two work just fine lol?!? anyways i ordered another master cylinder thinking that it has to be that? i put on a used one on new to me but used. it will be here in the morning so hopefully this solves my problem. its crazy that the brake line on the side of the master cylinder runs the passenger front and rear driver brakes and the front brake line runs the driver front and rear passenger brakes but thats the way it works on my car lol!! i also took ront brake line off of master cylinder and compressed both front and rear calipers and the fluid came out in a steady stream. i then put the front brake line back on the master cylinder and beld brakes then pumped up the brakes and they stuck again so it has to be that stupid master cylinder?? dont know what else it could be
Hi craneop2009,
Did you ever get your brake problem solved? Did you replace the master cylinder and did that fix your brake lockup problem? I am having the same problem with my left front brake on my 1992 Mitsubishi 3000GT VR4. I have already replaced the hydraulic unit (HU) and the proportioning valve, and brand new front Brembo Grand Turismo brake kits on both sides of the car. Pulled the ABS fuse, and this did not help either. All other brakes on my car work perfectly. It's just the left front brake that is locking up. Any advice will be much appreciated!
 

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Hi craneop2009,
Did you ever get your brake problem solved? Did you replace the master cylinder and did that fix your brake lockup problem? I am having the same problem with my left front brake on my 1992 Mitsubishi 3000GT VR4. I have already replaced the hydraulic unit (HU) and the proportioning valve, and brand new front Brembo Grand Turismo brake kits on both sides of the car. Pulled the ABS fuse, and this did not help either. All other brakes on my car work perfectly. It's just the left front brake that is locking up. Any advice will be much appreciated!
It's the master cylinder... It's always the master cylinder. If you look hard enough, you'll probably find at least 20 threads on this site for the same thing.
 
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