Brake Pedal Fade from Overheating??

[MUDZLLA]

I found with a 1HDT it was better to shift into neutral on long descents otherwise you were fighting the engine torque at idle. Also kept an eye out for suitable trees to ram if the brakes failed. Not sure if the gas trucks have this problem of driving the truck forward at idle with an 442F. But this is on cutlines in the mountains not roads.

 

[z531519]

I found with a 1HDT it was better to shift into neutral on long descents otherwise you were fighting the engine torque at idle. Also kept an eye out for suitable trees to ram if the brakes failed. Not sure if the gas trucks have this problem of driving the truck forward at idle with an 442F. But this is on cutlines in the mountains not roads.

i do the same, especially for very steep descent that i need to crawl at 1mph. I also tend to loose vacuum by 3rd brake pump when fighting the engine so neutral it is, ready to switch to gear in case things don't go as planned. Same backing up a steep hill.

It's even worse when the engine bay is hot, AC-on, and dealing with sticky throttle cable when it gets real warm (i'll fix that someday).

 

[JaketheSSnake]

Interesting. Myself and 2 other 80s were having problems with this over the weekend. It was hot, my ac was kicking out and my brakes went to s***. 4 low was pushing through them so bad I had to shift to neutral a few times. I'm sure a fluid change is in order, going to delete the abs and do that as well.

 

[musthave]

Did another fluid change, 100% new. Tried to make the problem again and succeeded. It atill boils and when it foes, no stopping. Or very little. Hot engine, hot temps outside, heavy braking = brake fluid boils quickly. Experimented on Imogene pass. 4 low, in low from the beginning probably prevents it. How to not have boiled fluid. Hmmm. I’ve got some ideas, we’ll see.

 

[60wag]

I just had my first significant loss of brakes. Driving I70 eastbound from the Eisenhower tunnel down to Georgetown, yea I know is a bit steep and I was towing a 4000lb travel trailer. Traffic was moving very slowly, I was in 2nd or 3rd gear and lightly using the brakes, couple seconds on and 10 seconds off, def' not heavy braking. Earlier this year I redid the brakes, new rotors, pads, soft brake lines and a complete fluid flush.

I get down to Georgetown and am noticing really mushy brakes, like pedal to the floor mushy. I'm right in front of a large chain-up area for semis so I pull over and stop. I can easily push the pedal to the floor both with the engine running and shut off. With a couple pedal pumps, I can build pressure and get some feel of a normal brake pedal. I figure the fluid is too hot and needs to cool. Waited 20 minutes or so, the pedal feel got a bit better but still easy to push it all the way to the floor. At this point, I'm wondering if the master cylinder failed rather than simply boiled fluid?

Since I can get some braking by pumping it, I make the choice to get back on the road. Its another 60 miles to get home. The whole drive home, the brakes appear to slowly improve, still a bit mushy but better. The only other time I've overheated brake fluid was on a motor cycle. Five minutes of cooling and everything returned to normal. The length of time the Cruiser needed to cool seemed to indicate something wrong besides simply hot fluid. I'm inclined to replace the master since its never been done, (186k on the truck).

What do you think?

 

[musthave]

I just had my first significant loss of brakes. Driving I70 eastbound from the Eisenhower tunnel down to Georgetown, yea I know is a bit steep and I was towing a 4000lb travel trailer. Traffic was moving very slowly, I was in 2nd or 3rd gear and lightly using the brakes, couple seconds on and 10 seconds off, def' not heavy braking. Earlier this year I redid the brakes, new rotors, pads, soft brake lines and a complete fluid flush.

I get down to Georgetown and am noticing really mushy brakes, like pedal to the floor mushy. I'm right in front of a large chain-up area for semis so I pull over and stop. I can easily push the pedal to the floor both with the engine running and shut off. With a couple pedal pumps, I can build pressure and get some feel of a normal brake pedal. I figure the fluid is too hot and needs to cool. Waited 20 minutes or so, the pedal feel got a bit better but still easy to push it all the way to the floor. At this point, I'm wondering if the master cylinder failed rather than simply boiled fluid?

Since I can get some braking by pumping it, I make the choice to get back on the road. Its another 60 miles to get home. The whole drive home, the brakes appear to slowly improve, still a bit mushy but better. The only other time I've overheated brake fluid was on a motor cycle. Five minutes of cooling and everything returned to normal. The length of time the Cruiser needed to cool seemed to indicate something wrong besides simply hot fluid. I'm inclined to replace the master since its never been done, (186k on the truck).

What do you think?

I think brake fluid boils quickly. Once the calipers and rotor are hot, they don’t cool off very quickly. I don’t think the master cylinder Will prove to be the issue. Perhaps a brake booster.

 

[60wag]

So having driven the truck around this week, the brakes feel completely normal again. Master cylinder appears fine. I am still surprised that I managed to boil the brake fluid and how long it took for the system to cool to the point that the brakes worked well again.

That said, has anyone moved to DOT4 or even DOT 5.1 fluid in an 80? The DOT3 I have in there is less than a year old, so it should be close to its dry rated boil temp. The DOT4 should gain another 50F before it boils. I was at 9000ft when this happened, not sure how much that could effect the boiling point, don't think its significant.

 

[landtank]

How are you determining that the fluid is boiling?

 

[60wag]

The pedal went to the floor after descending a steep road with a heavy trailer behind me. I managed to pull over and waited 20 minutes while occasionally pressing on the pedal. It continued to go to the floor but I found that I could build pressure with several repeated pumps. Once the system fully cooled down, the old pedal feel came back. The level of the fluid in the reservoir never dropped. I should keep the IR thermometer in the truck to measure the caliper temp if this happens again.

 

[musthave]

The pedal went to the floor after descending a steep road with a heavy trailer behind me. I managed to pull over and waited 20 minutes while occasionally pressing on the pedal. It continued to go to the floor but I found that I could build pressure with several repeated pumps. Once the system fully cooled down, the old pedal feel came back. The level of the fluid in the reservoir never dropped. I should keep the IR thermometer in the truck to measure the caliper temp if this happens again.

So, on the last leg of my trip east, I purposefully used the brakes very hard one afternoon. Finally, they started to give out. Brake fluid temperature was 199. Yes, I believe the brake fluid is boiling.

I’m sorry to hear that the master cylinder isn’t it, but I didn’t really expect that it would. Brake booster would be my next step, but again, with a 100% new brake system from beginning to end I still have the issue. Brake fluid boils. YMMV

 

[landtank]

The pedal went to the floor after descending a steep road with a heavy trailer behind me. I managed to pull over and waited 20 minutes while occasionally pressing on the pedal. It continued to go to the floor but I found that I could build pressure with several repeated pumps. Once the system fully cooled down, the old pedal feel came back. The level of the fluid in the reservoir never dropped. I should keep the IR thermometer in the truck to measure the caliper temp if this happens again.

this sounds to me like loose rear wheel bearings with binding caliper slide pins. On rough decents the wobbling rotor would cause the caliper pistons to open because of the binding slide pins and I would have no pedal at all. Jack up the rear axle and try pushing/pulling the tire in and out to check for play.

 

[landtank]

So, on the last leg of my trip east, I purposefully used the brakes very hard one afternoon. Finally, they started to give out. Brake fluid temperature was 199. Yes, I believe the brake fluid is boiling.

I’m sorry to hear that the master cylinder isn’t it, but I didn’t really expect that it would. Brake booster would be my next step, but again, with a 100% new brake system from beginning to end I still have the issue. Brake fluid boils. YMMV

boiling temperature of DOT 2 brake fluid is 374*

 

[musthave]

boiling temperature of DOT 2 brake fluid is 374*

At what temp should / would it start to effect braking?

 

[landtank]

not sure but your rotors would be glowing.

 

[musthave]

not sure but your rotors would be glowing.

Second time I experienced it, I poured a few gallons of water on the rotor, all turned to steam. Super hot.

 

[landtank]

water turns to steam at 212*, I don't think your brake fluid is boiling.

 

[musthave]

I need to get it to complete failure again and get temps of rotor, line, and fluid. From the pedal to the wheel it’s 100% new OEM. Might be a while though, Florida is too flat.

 

[landtank]

You might go to my site and look at my brake up grade. Not for the upgrade but there is a document on there that outlines how to fully inspect the brake system on our trucks.

And if you have lifted the the truck have you addressed LSPV?

Not doing so puts all the braking force on the front wheels alone which means all the heat as well

 

[musthave]

I’ll have a look at the doc you have. Thank you

 

[60wag]

Took the opportunity to rotate the tires so I could inspect the brakes and hubs. The wheel bearings all felt good. Rotors and pads look good. Def' no leaks. I will look at the LSPV lever and the docs. I don't think it was adjusted after the OME 2" lift went in, 14 years ago.

I've got a Tekonsha brake controller for the trailer. I did turn up the braking level and the boost from 1 to 2 after the loss of brakes event. I need to play with the brake controller more to be sure that it is doing what it is supposed to.