Cannot fully tighten ATE master cylinder plastic reservoir cap-is that normal?

[Ricardo]

The cap on my ATE master cylinder plastic reservoir cannot be tightly tightened. When I try to tighten it tightly (moderately tightly) it clicks past what first seemed to be the end of its travel, i.e. it feels like the big plastic threads where the cap screws onto could be stripped. However, when I look at the threads both on the neck of the reservoir and on the inside of the cap (which appears to indeed be the correct ATE cap), all those threads look ok. Because of their large size they are easy to see.

So my question is: is this kind on impossible-to-tighten-tightly-cap normal on the ATE plastic reservoir, maybe some kind of protection against overtightening? And does that plastic reservoir not need to contain that much pressure, because it is only a reservoir, the actual high pressure remains only inside the master cylinder? The brakes seem to work ok, but I am using them gingerly til I can understand this. The previous owner told me it had been like that and the brakes worked fine.

1966 230SL

[ja17]

Some of the new reproduction plastic reservoirs are very slightly smaller than the originals. In this case the original caps can slip pass the threads on the reservoirs. For those who are familiar with molding plastics and metals, if an original part is used for a pattern for a mold, then the finished part will be very slightly smaller from shrinkage after the molded material cools and shrinks. To prevent this manufacturers make a pattern from scratch and make the pattern ever slightly larger than the designed finish part. In this way after the finish molded part cools it shrinks to the exact designed size and the cap fits as it should! 

[Ricardo]

Joe you are exactly right, now I see it. The cap looks original, not so sure about the reservoir. So the other aspect of my question: is it ok to keep driving with the reservoir cap only quasi-tight like that? As far as I understand, the connection between the master cylinder and the plastic reservoir operates like a one way valve, letting brake fluid into the master cylinder if necessary, but not the other way around. And the cap closes tight enough to keep out dust, and the brakes feel fine when using them.

[FGN59]

There is no pressure in the brake fluid reservoir, so as long as the cap does not unscrew itself with the vibrations of the car (engine, road bumps, etc.) you should be fine.

The only issue is that the brake fluid is hydrophilic (it likes to absorb water, in this case humidity rather), so your fluid in the reservoir would ‘deteriorate’ a bit faster than normal from humidity absorption if the cap is not really tight. You could try to wrap a little bit of tape around the threads on the neck of the reservoir to make it tighter (it would be invisible from the outside).

Just my 2 cents on this issue. This is what I would do.

[Pawel66]

I will add 2 cents so that you have 4 cents altogether... See if the liquid is not splashing through the cap - it is very aggressive, will eat through paint to the body - very dangerous.

[stickandrudderman]

The only issue is that the brake fluid is hydrophilic (it likes to absorb water, in this case humidity rather),

Pedant mode on:
It's Hygroscopic.
Pedant mode off.
 

[FGN59]

Always glad to learn something, and thanks for the tip (and for making probably a few others like me wiser too).

So if I got it right this time, brake fluid is hygroscopic, a sponge is hydrophilic and a drunk is hydrophobic? 

[ja17]

Since the cap has an open vent hole, I would not worry about it being slightly "less tight" unless it leaks fluid.

[Ricardo]

I just used a "Bremsflussigkeitstester" (thick pen-like device with a pocket clip like a pen) to test the brake fluid water percentage and it shows green "OK" approx. 1% mark, and this is the first such test in over 3 months, so if any humidity is getting in it is very little and very slow. Extremely quick and easy to test, took 2-3 seconds, the tester is so handy to keep in the glove compartment, seems worth also checking this for peace of mind whenever checking the oil.

But thinking about it now, does the humidity percentage in the brake fluid inside the plastic reservoir reflect the actual humidity of the main volume of the brake fluid that is circulating in the master cylinder, brake lines, calipers and drums?

Riccardo, 1966 230SL

[stickandrudderman]

But thinking about it now, does the humidity percentage in the brake fluid inside the plastic reservoir reflect the actual humidity of the main volume of the brake fluid that is circulating in the master cylinder, brake lines, calipers and drums?

Riccardo, 1966 230SL

Yes it does contaminate the entire volume of the brake fluid.
The presence of moisture in the brake fluid will lower the boiling point of the fluid which can cause brake fade or even total failure if the brakes get too hot such that the fluid boils.
This is why brake fluid is changed every two years.