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Old 03-21-2009, 07:37 PM
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front disc locking up

I have a front disc rear drum setup with a disc drum master cyc and booster. I have been driving the car for about 50 miles now with no problems. I took it out today and notice when I let off the gas the car would slow down fast. It kept getting worst and on my way trying to get back home it was all the motor could do to move the car. The front brakes were really smokimg. I made it and it was almost on fire. Now I'm taking BOTH front disc brakes were locked up. I let it cool down and jacked up the front. After it cooled you could turn the wheels but as soon as I cranked it up it locked down. Being that both fronts are locking up I would think it has to be in the master or booster. Also the master and booster are under the floor but I have the check valves on both front and rear and the right size on each. Like I said it has been working fine. Bad check valve? Master cyc rod sticking or piston? I did find a week soft spot in the vac hose to the booster. Could be closing under vac? Would that cause the lock up? Thanks for any help

Randy

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Old 03-21-2009, 08:02 PM
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all stop no go

i would suspect the master cyllendar but almost any of your listed suspects could be the culprit. its hard on the pocket to diagnose with parts.
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Old 03-21-2009, 08:53 PM
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If it locked back up when you started the car it must be vacuum booster related. I would disconnect the vacuum hose to the booster temporarily and see if that fixes it.
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Old 03-21-2009, 09:25 PM
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First thing I'd check is the freeplay at the pushrod that goes from the pedal to the booster. If you have about 1/64" or so of freeplay, it's most likely your booster airvalve...you could also try adding a pedal return spring too, sometimes the small boosters don't get the pedal all the way back up.

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Old 03-22-2009, 12:21 PM
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locked brakes

I have went over the system again and the only thing I found was a solf spot in the vac hose to the booster. It felt like it was almost closed. I replaced it and it is now its not locking up. As a matter of fact I don't have much brakes at all. The pads got so hot I guess it messed them up. Have to drive it some more to be sure but I wouldn't think the vac booster if it was not getting enought vac would hold the brakes so tight the car would not move.

Last edited by loctite; 03-22-2009 at 03:54 PM.
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Old 03-22-2009, 04:28 PM
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brakes

If the brakes are dragging, as they hotter and hotter eventually the fluid starts to expand in the caliper just from shear heat build up, the longer you run the hotter it gets and they expand even more.
When they cool off the wheel will turn again.
All this palys hell with the fluid, and the check valves, and generates air bubbles if the fluid gets hot enough to bubble.
The pads will be scorched and worthless if they get too hot. They get glazed, and need to be tossed. Especially if the fluid has expanded enough to get out out on the pads THEN heat up.....bad,bad,bad.
Just went thru this with a big touring motorcycle.
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Old 03-22-2009, 05:22 PM
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I would suspect the push rod between the master cylinder and power booster is too long. This will cause the front brake pads to stay engaged while hot.

This can be checked by loosening the master cylinder mounting nuts while the front brakes are locked up. If they free up the push rod is too long.

The only other problems I have found are the brake pedal free play height adjustment, and/or a stuck proportioning valve.
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Old 03-22-2009, 06:43 PM
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locking brakes

Thanks for all the help! The brakes are working fine as of now. I put new pads on. They worked fine till the vac hose closed up and cut the vac off or low to the booster.
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Old 03-29-2009, 10:25 AM
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Quote:
Originally Posted by loctite
Thanks for all the help! The brakes are working fine as of now. I put new pads on. They worked fine till the vac hose closed up and cut the vac off or low to the booster.

Thats a new one for me and good to know. I know you have solved your problem but it reminded me of one I had on my 56 Pickup. I had just finished the truck and noticed the same thing, only it was just the right front. I stopped for a signal and it was all the engine could do to move that truck with the drag on the front wheel. I waited 15-20 minutes and it cleared up but I knew something was wrong so I headed home. By the time I got there it was locked up again. Turns out that I had the headers, exhaust and cut outs installed last and they were too close to the steel line running to that wheel and was heating the fluid up. I re-routed the line and no more problem
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Old 03-31-2009, 12:34 PM
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I had a '76 Blazer parts chaser- $400 deal w/bad tranny.

Anyhoo, one day I was pulling a travel trailer to a storage facility, and had the same dragging brakes syndrome.

I though it had to do w/the trailer. It turned out to be the front hose drivers side was being pinched by the steel bracket that the hose runs through, that's mounted to the A-arm.

This Blazer was from PA originally, rust had built up on the inside of this retainer to the point that it was almost like doubling over a water hose to shut the flow off!

Took the bracket off and used tie wraps, all was good.
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Old 04-01-2009, 08:39 AM
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Sorry, but there is no way that a plugged vac hose to the brake booster can in any way, shape, or form cause the brakes to drag. At most, it will REDUCE the amount of brake boost. That's it. Now a defective inlet valve on the booster (the internal valve on the input pushrod from the pedal) CAN cause the brakes to drag. A more common problem is that the lining in the rubber hoses to the calipers can collapse, causing it to act like a one-way valve. When were the hoses last replaced?
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Old 04-03-2009, 06:58 AM
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Just posted same problem before I saw your post. I do not have power brakes. Both front brakes are also hanging up on mine, therefore I don't think it would be a hose. Could anyone confirm if residual valve is a check valve and if so could it be sticking?
Rick
PS Don't think your problem is fixed with vac. hose replacement, as posted earlier ... no way that will have any affect on brakes hanging up. Chances are that your problem will be back. HINT Carry a wrench to bleed pressure off calipers to get home.
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