I have a 1992 polo green convertible with approximately 51K on the clock and I have been having issues, somewhat regularly, with brake lock up. When initially starting up from cold, the brakes are fine, however, the pedal goes a bit deep when stopping. This corrects as a few miles are put on the car and the pedal then feels normal. It is a six speed and when engaging the clutch the car will freewheel easily. However, you never seem to know when this will happen but when stopped or even in traffic the brakes may suddenly start dragging and gradually lock up. I've been in this situation a number of times and have read about others who have had to wait until the rotors cool down in order for the brakes to release their grip. This is not the case here though. I have managed to correct the issue when this happens by shutting the engine down, restarting it and then applying the parking brake and then releasing it and this somehow frees up the brakes and I'm on my way. I can continue to drive the car with no issues and no further lock ups, so far. I have had the car up on a lift and wheels off and there seems to be no issues with the calipers or rotors. The brakes have been bled and no apparent air in the system. After the most recent episode when I returned home, I opened the master cylinder reservoir after the car had cooled down for a couple of hours and did notice that there was a ring of small bubbles around the inside rim similar to what you might get in a carbonated beverage. Would this be a symptom of air hiding in the system somewhere else or just the fact that the fluid may have heated to such a point when the brakes locked up that could have caused it to overheat? The other thing I don't understand is how the parking brake can free up the regular brakes if they are locked up since both work independently of each other, one by hydraulics and the other by cables. Does anyone have any ideas where to go next to solve this mystery? A little back round, I am only the third owner of the car and as far as I know the original master cylinder and booster are still there.

 

Possibly internally collapsed flex lines to the calipers.

 

 

me 3

 

Ok, thanks much guys! It's a start. I'll have to get it back on the lift and see what we find there. Internally collapsed flex lines? Sounds like an automotive version of arteriosclerosis preventing the free flow of brake fluid through the system.

 

The insides swell up and restrict the movement of fluid, and they can even fully swell to the point where they are completely blocked up....

 

If it was a flex line the car would pull and having more than one bad and do this is not impossible but very rare.

But I'm going to tell you what is probably wrong as I have seen this many times.

Due to heat, the actuator rod in the brake booster is pressing against the master cylinder causing the brakes to be slightly applied. The rod is expanding from the heat. There is not enough clearance between the two and need to be adjusted.

A check of this is when it is happening, jack up one of the front wheels and check it. Then unbolt the master (Usually a few turns) pull it back, and see if the wheel frees up. If it does, the actuator rod need adjustment.

 

Interesting theory beside the flex line one mentioned earlier. Yes, when this happens it seem like all four brakes lock up. One prior time when it happened , I got out to feel all four wheels and they were all hot to the touch which would indicate to me that all four wheels were affected. In thinking about the earlier solution a bit more, I would think that having all four flex lines fail at the same time would be somewhat rare. The car does not pull to either side when this happens. Your theory of the brake booster actuator rod being the problem would seem to make more sense since all four wheels are involved. The thing that I don't get is how the parking brake would cause the brakes to release when they lock up. One is hydraulic and the other just works off of a set of cables. The heat from the lock up would not dissipate to cause a release that fast. Trying to check this by lifting the car after it happens could be problematic since you never know when this is going to happen and it could be an issue if you're in traffic with no place to pull over. Adjusting the actuator rod is a heck of lot cheaper than going for an entirely new set of brake lines.

 

It's not a theory or a one of a kind out in left field. It's a reality that I'm familiar with for over 15 years.
Understand you can't do it in the street, got to be near home and ready to do the test quickly. You have to build up heat in the engine bay so it all gets hot so that might take some time.

Or you can just adjust the rod 1/8 inch in and go from there and see if fixes the problem.

 

If the lines are original just replace them….bonus will be a full fluid flush will occur too…

Even if this is not the issue the lines should be replaced anyway as they are critical and very old if original.

I’ve seen a hose go bad like this on other cars…calipers replaced etc and it turned out to be a line…

I agree if it was significant braking force it should pull… also if it’s dragging one wheel you should be able to tell just by smell.

pcolt’s theory is interesting

let us know what you find

 

Got it! However, if heat to the booster is what is causing the output actuator to partially apply the brakes, why would the parking brake release the brakes? Thats what throws me. The other thing is once it happens and I get the brakes to release, the car drives normally. The engine is still generating heat but the brakes aren't locking up unless maybe the output actuator is only very slightly being applied to the rotors and not appreciably being detectable while driving? I've been digging through the manual and have looked at the section that covers the booster. It doesn't appear that you can adjust the output actuator. The only thing it says is that if it's out of kilter then either disassemble the booster and repair it or replace the whole booster. From what you are saying this is starting to sound more and more like the booster and not the flex lines since all four wheels are affected. Thanks!

 

The rod is adjustable as I have done it many times on boosters. Also when a new booster is installed it needs to be checked and adjusted if necessary.
Have not done it in a while but you have to take the master bolts off. Then pull the master back all the way and secure it with a (bungee) cord.
Then hold the rod with long vise vice grips and turn the nut. (Can't remember if the vice grips goes on the nut or rod but will be clear when you look at it).

OK.......so having said all the things I have said, I am really confused about the parking brake thing you described. Makes no real sense to me at all and can't see the relationship to it.
Usually if the actuator rod is to long, it does not release till it cools. So that's conflicting to the situation.
But if you can I would still do the jack up thing I described as a way of troubleshooting and isolating the cause.
Definitely a screwy problem given the circumstances.

 

Or it’s just a sticking caliper/caliper piston in the rear, and pulling the e brake breaks it free. Also, exercising the piston without hydraulic pressure in the lines (ie foot off brake pedal) might release pressure from a collapsed line. I.e. pulling the e brake would move the piston outward and release the pressure in the line between the caliper and the blockage. This would allow said blockage to “open” and fluid to flow back out as it should when e brake released. The back pressure is what keeps the collapsed section collapsed, and once that back pressure is removed by pulling the e brake it opens enough. It acts like a one way check valve.

I would concentrate on the rear. This could still be one brake engaging… best thing to do is replace the rubber lines!!!

 

not sure if this is same issue, but i had an issue where asr/ abs light would come on all the time.

My solution was to make sure master cylinder is filled to the brim

even a little above the fill line. Seemed to fix it

——
obviously make sure air isnt getting into your system Nd that its well bled

 

Yeah, screwy isn't the word. If the brakes locked up due to overheating because the booster output actuator rod was in the wrong position, the brakes should not let up until they cool off but that's not the case here. The parking brake will get it to release after shutting the motor down, restarting and then applying the parking brake and then releasing it. I then was able to drive around at various speeds without any issues. Strange! I brought it in and had it up on a lift, and the guy I had go over it who has worked on old Corvettes and owns a few could not find anything wrong after taking it out and riding it around. Figures, when I need it to happen, it doesn't!
Getting back to what you're saying about adjusting the output rod, What nut are you talking about once the master is disengaged? From looking at the exploded diagram in the manual, the output pushrod is held in place by a seal assembly and a retaining ring only. Looks like there is a similar set up on the input side. Could the master possibly be at fault here rather than the booster? As I mentioned earlier, the last time this happened I opened the reservoir cap a couple of hours later and noticed a rim of small bubbles around the meniscus on the surface of the fluid in the reservoir. I wonder if this might have been from overheated fluid backing into the master when the brakes were locked up?

 

Ok, adding the last two comments and everything we've talked about previously, it seems we've narrowed this down to replacing the original brake lines which may have an internal collapse or looking at the output pushrod in the brake booster as the culprit. The last post seemed to provide a plausible explanation as to how the parking brake would get to release the brakes. This would have me leaning toward starting with the brake line replacement as a start. On that note, steel or stainless replacements for the old rubber?

 

I’d use dot certified braided stainless lines to replace the rubber lines…they are readily available online

My only other thought is an ABS pump issue but I wouldn’t think it would behave like this…

 

Go with the braided lines I have them on my 94. They are advertised to make the pedal more firm and take some sponginess out, whatever. I was chasing a problem years back and replaced the lines. Didn't fix it, but glad I replaced them anyway. I don't think its ABS either. The earlier 80s cars did have ABS problems clogging with no fluid flow but I think your good on this. And I never used any special tools to make a slight adjustment.

So here is a poor picture, all I got presently in my pictures.
You hold the rear larger shaft and turn the inner screw (nut) to adjust, If you wind up going there.

Here are a few links to see also

 

Great! Thanks for those links and the photo! Always good when a visual is available. I was leaning to the braided stainless myself. The cost isn't all that much more than plain steel. I was also wondering if it would be worth it to just replace all of the lines with S/S as well? I did a quick look see under the hood and rear to see if I could spot any irregularities in the hoses but didn't find anything that jumped out at me. There didn't appear to be any cracking or leakage but who knows what's going on inside of them. From what I've been reading, I agree that this doesn't sound like an ABS issue at all. I'll start with replacing the hoses first since they are originals and probably should go anyway and then move on to a booster output rod adjustment if that doesn't fix the issue.

 

The only rubber lines that are part of the brake system are directly attached to the calipers. I would not touch the current solid metal lines unless you have good reason or obvious corroded/leaking lines (which would be rare). The solid tubing should be fine…

my suggestion is to do one line at a time, bleeding after each line is replaced. This minimizes your chances of running the master low and introducing air up stream