Brakes overheating

[mauro12]

Hi guys  , I wanted to tell you my experience after a nice trip with 3 pagodas , 4 r129 and some r107 Mercedes . We were on a nice tour around mount Etna ( Sicily ) , the day was very hot , around 32 degrees and we were descending from and altitude of 2000meters to the nearest town around 400m above sea level . After 30km of a very down hill road full of braking , after about 40 min , my brake pedal travel was almost on the floor , probably due to overheat of brakes , the speed was also very low , maximum 40kph . We almost all  had the same problem, both pagodas and 2 r129 and 2 r107. After a few minutes the brake pedal travel  came back like before . The fluid reservoir had only  few drops of leak but the level is almost at max .
I’ve never experienced such a thing but I believe today we asked too much from our car . Have you ever experienced something similar ? What do you suggest to do ? Is the first time for me to drive in a such high altitude . Also the engine temperature raised a bit  during the way to the top , around 35 km up hill . Usually is very stable at 80 degrees Celsius but this time it raised to 90-95 Celsius .
Thank you

[Cees Klumper]

I've not had this issue fortunately - since you have a 5-speed, you could try engine braking more in future simliar situations? Perhaps you already did that, but that is one technique that is generally recommended. I even see it on road signs in the Jura mountains and similar, recommending truck drivers to 'engine brake' on long descents.

[DaveB]

Hi Mauro, it is common, they get smoking hot. I’ve ruined brake pads that way (glazed the surface). Best thing is to use a lower gear so you can brake less.
Oh I see Cees already posted similar advice.

[mauro12]

Do you suggest to replace the brake fluid ? It should be replaced every 2 years but I believe almost none follow this rule .
This phenomenon is called vapor lock. I’m sure that even with new fluid , after 30km of braking even the new fluid could have reached the boiling point .

[stickandrudderman]

Two things:
1. Try to use engine braking by holding a low gear to moderate the speed instead of the brakes.
2. Brake fluid is hygroscopic. That means it absorbs moisture from the atmosphere which lowers the fluid's boiling point. You can imagine what boiling brake fluid does to the feel of your brake pedal and hence why, when the brakes cool and the boiling stops, your pedal returns to normal.

This is why brake fluid should be changed every two years, regardless of mileage.

[Pawel66]

mauro, please take a look:

https://www.aa1car.com/library/bfluid.htm

"Almost none follow this rule" - well, I do. I would do particularly living in mountain area. But even in regular driving, during emergency braking event the boiling point is reached very quickly... Lot's of kinetic energy needs to transform into heat.

[mauro12]

I’m not saying none but I believe that maybe 10% do replace once per year . Do you know how many liters the system takes ? Better to use  dot 4 than 3 right ?

[Pawel66]

As per our Technical Manual you need 0,5L of DOT 4 brake fluid. You will probably spill some a bit during exchange/bleeding.

I am using DOT 5 bought from Mercedes, but that is just me.

[MikeSimon]

Pawel: You can only use DOT5 after you make sure all components in your car's braking system are compatible. If you ever had DOT3 or 4 in the car and change to DOT5, you are inviting serious problems. Our cars were not made for DOT5. Some seals and hoses react aversively to the silicone based fluid. As DOT5 is not hygroscopic, the moisture in the fluid (which you will have) will settle on the lowest point in the components and cause corrosion. The only real advantage DOT5 has, is that it does not attack paint. Other than that, DOT4 or DOT5a is the best fluid for most vehicles out there. Including racers.

[mauro12]

Guys, do you know if you have to buy a new brake fluid reservoir  and the fitting kit during the master cylinder replacement?
Can you re use the old reservoir with the new master cylinder?
Since they are both attached, I assume that is very easy to damage the plastic reservoir when you remove it from the cylinder .
Mine is completely yellow after 60 years .
I’m wondering if it was white when new  or light yellow . Thank you all

[Pawel66]

Pawel: You can only use DOT5 after you make sure all components in your car's braking system are compatible. If you ever had DOT3 or 4 in the car and change to DOT5, you are inviting serious problems. Our cars were not made for DOT5. Some seals and hoses react aversively to the silicone based fluid. As DOT5 is not hygroscopic, the moisture in the fluid (which you will have) will settle on the lowest point in the components and cause corrosion. The only real advantage DOT5 has, is that it does not attack paint. Other than that, DOT4 or DOT5a is the best fluid for most vehicles out there. Including racers.

Mike, thank you. I remember I was checking this some years ago when I started - there is no single seal that would not have been replaced, the lines are copper. I will ask again, thank you for the point. I remember discussion about it here on the forum.

[mauro12]

Guys , during this process i guess it's worth to replace the brake rotors too. I'm wondering what to buy , ate or febi ? febi is well known german brand and the price is half of ate. From what i can see the ate rotors seems to have better coating as a rust protector . Do you have any suggestion about what to buy? My winter project is to replace the master cylinder , lines , discs and fluid . Calipers are ok as i drive the car very regularly , almost once a week and i've never had any issue with those. Is there anyone among you who has mounted febi discs ? thank you

[awolff280sl]

If you are replacing the rotors, you may want to consider vented rotors (off of a W108 if I remember correctly). They may help with the overheating. But you would need to change the calipers as well.

[mauro12]

This would be a nice idea but replacing the calipers is too expensive at the moment. It would be great if a company could make vented disc suitable for pagoda with the standard calipers .