Thanks. I suspect something like that is going on. Ordered some vacuum tubing from Moss. Plan to replace what I have to see if that solves problem. There was a discussion of vacuum leak problems in an old thread on this forum.
Interestingly, I think my problems appear when engine is running somewhat hot. It was 96 when brakes started locking up and I had been running for an hour at 75-80 mph. Temp gauge read on High side of normal, needle pointing at the “l” in normal. Not quite sue what temperature and vacuum correlation would be but it might be real.
The car was indeed running hot and for about an hour on the highway. Not sure about temperature and this brake drag as well, but it is interesting.
Temperature often aggregates a lack of free play issue.
Tom
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Temperature is a red herring. The condition of that brake bottle is probably indicative of problems elsewhere. An end to end cleanup is in order. Rebuilding the master is probably the best next step.
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Been kind of side tracked with a long hiking trip but finally getting back to trying a few things on the brake locking problem. It was suggested that I start replacing the vacuum lines and it seemed like a logical start point would be the short segments attached to the master cylinder itself. Since I am rebuilding the clutch master cylinder, these hoses are a bit more accessible. But I soon found out that I am a novice in identifying the correct hoses. There seem to be two attached to the Master cylinder unless I am getting things totally confused. I have attached a couple of pictures of the hoses in question, at least as best as I can picture them. In the first picture, there is a short hose from the side of the master cylinder connected to a plastic cylinder at the front. The other hose is a bit more difficult to see so I took two pictures. One shows the hose heading under the master cylinder where it connects (but I cannot easily photograph.) the other picture shows it connecting to what appears to be a vacuum tank. I could use some help from the more experienced folks. Hate to mess this up too badly. I considered rebuilding the master cylinder but everything seems to work well except for the unexpected brake locking which I am convinced is a vacuum problem. Comments please.
Oops. I was going from memory and I guess I am getting old. The line in the first picture also goes to the tank but on the top rather than the front. I apologize for not checking.
I had a pick-up truck that did the same thing and it drove me crazy, I had to change the brakes every few months because I could not find what was the cause of the failure and those sad mechanics in the area had no clue. I finally had my neighbors son over, who is an airline mechanic that went directly to the problem. It was the brake servo hose that disintegrated internally and also the brake lines failed from using different brake fluids in the reservoir. The brakes were fixed and were as good as new after a few hour.
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The critical line is the one that comes off the plastic reaction valve. This is the line that is used to release vacuum from the back of the servo when the reaction valve opens. The other line to the master cylinder supplies vacuum to the rear of the servo when the reaction valve closes.
We know there’s vacuum leakage at the rear of the servo, because the problem is caused by higher pressure at the rear of the tank than at the front. We also know that the vacuum supply isn’t compromised, because if it was, then both the front and rear of the tank would be losing vacuum and the brakes couldn’t be locked. So the places to look for a problem are the release hose from the reaction valve, the reaction valve itself, the small piston in the front of the master cylinder, and finally the servo. It wouldn’t hurt to replace the rest of the vacuum hoses, but I don’t think you’ll find the problem there.
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Thank you. Will replace both of the hoses if I can get the one off that is attached under the Master. That is a tight space 
You’ll find the other hose goes to a T, which goes to the front of the booster and the dash supply tube. Since the T supplies both front and back, and the front is clearly holding vacuum, it’s unlikely to be the source of the problem.
Thank. Makes life a bit easier too.
Well, I am still in a bit of a fix. I replaced the short hose to the plastic reaction valve while I was replacing my clutch master cylinder. When I started car to check out clutch (which seems to work just fine), I realized that I couldn’t move the car and that the brake was locked in the full upright position! After shutting off the engine and sitting a few minutes, I could actually hear the breaks releasing (I am pretty sure that is what I was hearing.) I could then depress the brake pedal at least 1/2 way. I started the car and felt the brake pedal stiffen and lock at full upright position.
Should I replace the rest of the hoses? How about the reaction valve? Maybe the master? At the moment, I am thinking that replacing the rest of the hoses is the place to start. Suggestions?
So once again, the rest of the hoses are fine. You know that, because the brakes are being locked by vacuum. Remove the reaction valve and inspect. You’re looking for decayed gaskets. If it looks bad, replace it.
While the reaction valve is off, take notice of the small hole in the center of the steel mounting plate, Insert a small screwdriver there and push hard. Hold the screwdriver in and have someone depress the brake pedal. You should feel the secondary piston pushing the screwdriver forward, don’t try to hold it back. When the brake is released, you should be able to push the piston back smoothly. If not, the secondary piston is stuck. You can try to service it in place, or R&R the master.
If the master is working and the reaction valve sealing, then the problem is in the servo, and that 's a bigger project.
Thanks. Will look at reaction valve and try the screwdriver test. Hope this solves the problem.
What causes this problem is loss of vacuum at the rear of the booster:
Thanks. I replaced that hose at the rear of the booster (if it is the one that goes to the reaction valve.
Plan to look at reaction valve. Taking a brief trip and might not get to it for a bit.
Well, I had a few moments and decided to take off the reaction valve. The gasket is old but seems secure. Plan to replace it while it is apart.
However, when I tried to push the secondary piston with a screwdriver, I was unable to do so. Should I try tapping the screwdriver with a hammer? Or just give up and take off the master cylinder and press forward 
Really was trying to avoid that; very little room to work.
As I noted in an earlier note, managed to get the reaction valve off and couldn’t get secondary valve to move by pushing it. I am thinking of giving up and buying new MC. But I looked at the picture you sent showing the vacuum line entry points and realized that my MC doesn’t look anythng like the one you pictured. Did they change MC types for 69.
