Low brake pedal after highway driving

[rc51kid]

I have a intermittent low brake pedal. Most of my brake system is all new and it is in great mechanical condition. Around town the breaks are great, maybe a little spongy but stop like hell. No pulling, squealing shudder, no fluid loss or leaks or anything and have plenty of power. But if i cruise down the highway for a long time when i go to hit the brakes i feel like the peddle sinks lower before the brakes engage. My only thought is that i have some air or moister in one of the calipers. As it warms from the long drive it expands basically causing some brake fade. I tried bleeding with tubing on the calipers and the end of tubing submerged in brake fluid. It looked like it worked and i put about 1 liter of fluid through the system. But i do read that these trucks can be hard to bleed. Any ideas?

PS I was thinking about the booster not holding vacuum or something. But i addressed a booster problem in the past and a bad booster results in hard pedal with poor braking. Not at all what i have.

 

[CruiseOrlando]

Have you done the FSM test for the booster?

 

[rc51kid]

Have you done the FSM test for the booster?

Yes i did but i can check again.

 

[Landseer]

Driving at Wide Open Throttle (WOT) the engine produces less vacuum, with less vacuum the brake booster is less effective. For highway driving (throttle close to open) the brakes should be less effective. Old (very old) vehicles used to have windshield wipers powered by vacuum- at idle (high vacuum) they would go really fast but at high speed (WOT) they would slow down do a crawl. Not too hard to see why they switched to electric.

 

[rc51kid]

But if my booster was not working then wouldnt the brakes feel more like they do with the engine NOT running? Hard pedal with poor braking? That is not what i have.

 

[cartercd]

Did you ever resolve this issue? I am experiencing similar problems after sustained interstate driving. My internet search revealed that sustained driving could result in the master cylinder bore heating up and expanding, which could be enough to create a slight leak. It was recommended that you can pour water over the master cylinder to cool it and see if this resolved the problem. I haven't had time to do that experiment - maybe on next road trip. Most of my brake components / lines have been replaced in past several years and it has been bled extensively. The brakes work fine around town.

 

[alaniverson]

Reviving an old thread. This is happening to me, anyone know what could be causing this or the brake fluid to be heating up?
[L

 

[Tools R Us]

Are your wheel bearings tight, as in, no play?

 

[brianh699]

How old is your brake fluid? Brake fluid is hygroscopic (it absorbs moisture), this lowers it's boiling temp. These rigs are old, if you haven't flushed your system that may be your problem.

 

[leonard_nemoy]

I learn something new every day. Often times it comes from mud. I am working on my medical lab science degree and I have learned all about hydrophobic and hydrophilic, but hygroscopic is a new one. I love learning new words. Thanks ( fyi hydrophobic doesn't mix with water and hydrophillic does, nonpolar vs polar. And hygroscopic is the ability to absorb moisture from the air.)

 

[rc51kid]

Old thread. I can't even really remember what was going on when I started it. I think brakes basicly felt funny as the pads got really thin. I feel like with fresh pads, fresh rotors and fresh fluid it is no longer a issue. Basicly most everything in the brakes is newish now. I have a intermitant brake booster issue that only crept up a few months ago. So it is probably unrelated. But I have a new booster waiting to go in. I will look to see how dry the master is where it mounts to the booster when I have it apart.

 

[cartercd]

I had a similar issue. I used a infrared thermometer after driving on the highway and found one of my calipers was hotter than the others. This was likely a dragging brake pad which heated up the fluid, resulting in a soft pedal. I removed the caliper, exercised the pistons back and forth, put it back together, and no more problem. A wheel bearing which gets too hot could cause the same issue (as questioned in post #8).