XJ6 & XJ12 Series I, II & III 1968-1992

Brakes locked up after master cylinder replacement

 
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Old 07-24-2016, 11:22 AM
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Default Brakes locked up after master cylinder replacement

My 1987 Series III XJ6 was stored for about a year when I went to start it.

After a few cranks, it started right up and ran fine. Only problem was when I pushed the brake pedal, it went right to the floor!

It seems the reservoir was dry but I couldn't see any evidence of brake fluid on the ground.

I had it towed to my local shop and they found the seals between the master cylinder and the fluid container above it were leaking so they put in a new master cylinder along with a new brake booster that I had been meaning to change for some time (the only one had a tiny vacuum leak in it).

This all went fine but when we went for a test drive, I noticed the car had a lot of rolling resistance as if the brakes were still on. We jacked up the front to find the front wheels would not spin while in the air!

Never seen this problem before. Could something have gone wrong with any adjustments of the master cylinder/brake booster that would cause this? Or, could the front calipers just be rusted enough that they don't retract?
 
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Old 07-24-2016, 11:54 AM
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Originally Posted by e21pilot View Post
Could something have gone wrong with any adjustments of the master cylinder/brake booster that would cause this? Or, could the front calipers just be rusted enough that they don't retract?

There's an easy way to diagnose.

Open the bleeder on one of the affected wheels. If the wheel releases and becomes easy to turn, the problem is with the master cylinder/booster/pushrod assembly.

If opening the bleeder makes no difference, the problem is with the caliper.
 
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Old 07-25-2016, 10:52 AM
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If by following mikeys directions the brakes free up when undoing the bleeder, dont jump right at the master cylinder, another culprit can be the rubber brake hose, these can fail and only let fluid pass in one direction. To eliminate that as the issue, pump brakes until the wheel wont spin again then undo the union where the hose meets the brake line, if wheel does not free up the issue is in the hose!
Good luck!
 
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Old 07-25-2016, 11:06 AM
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OP stated that both front wheels were tight. The chances that both hoses failed simultaneously is remote.
 
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Old 07-25-2016, 12:03 PM
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Unlikely, but still a possibility, strange things happens when a car sits!!
 
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Old 07-25-2016, 07:32 PM
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The push rod in the booster needs to be checked for length to ensure it does not hold partial brake pedal pressure to the master cylinder. also check for brake pedal free play at pedal push rod. Just a couple of ideas that may help.
 
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Old 07-26-2016, 09:54 AM
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Way back when a couple of kid's big brother acquired a raggedy early 30's REO Coupe. Today's collectors would salivate. Then, it was merely a junky old car.


So, big brother and his little brother decided to make a "strip down".
Body piece after body piece was removed and then "tested' after each demolition.


I got to ride on a few of those tests. When the "big" six lost it's muffler, it was really fun.


There was a hitch though. It was one of the cars that got hydraulic brakes early on. One or more wheels would begin to drag until something had to be done.


Big brother solved that. A small 3/8" "spanner" in his pocket. Just pop a bleed screw on one of the front wheels and things would be free, for a while...


Why, none of us, including big brother had the slightest. did it matter? NOPE!!


Collapsed flex hose most likely....


Decades later, my 69 Toyota FJ40 Land Cruiser developed that malady. Recalling early days, I limped along with a spanner. Metric this time.


I broke it down into two causes.


1. Water contaminated brake fluid. Steam vapor lock. Bad master cylinder cap replaced.


2. The critter had weird brakes. Two cylinders for each of it's drum brakes. Corrosion. Honed and new seals. Linkage cleaned and lubed.


Fixed. One or both caused. Dunno.


Carl
 
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Old 07-26-2016, 10:31 AM
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Originally Posted by JagCad View Post

1. Water contaminated brake fluid. Steam vapor lock. Bad master cylinder cap replaced.

Nope. If water were to boil, the pressure would push the fluid backwards and vent into the master cylinder reservoir. Not possible to get 'vapour lock' unless the MC pressure relief port was blocked but then the brakes would drag anyway, steam or not.
 
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Old 07-26-2016, 10:46 AM
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One down. Good for the archives.


But, corroded brake cylinders because of contaminated brake fluid?


Trivia;


The earliest Duesenbergs used hydraulic brakes. Water was the first effort!! Should work, setting aside the corrosion issue.


Carl
 
 
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