All,

I've never heard of this before but im wondering if anyone else has this issue.

When cruising for moderate/long distances at "high speed" (85+) my brakes seem to back off or something of the sort and the pedal needs to be pumped before becoming firm again. I have inspected all visible lines and have not found any fluid leaking or other visible damage. I can see in the fluid resivoir a line of dirt above the fluid line showing that the fluid level is fluctuating about 3.5mm. I have crossdrilled slotted rotors and i am wondering if the excessive high speed airflow causes the pads to back off from the rotors. I have also contemplated that it may be thermal expansion related but have pretty much axed that theory in its singularity.

If anyone can confirm this suspision or offer an alternative explanation i would greatly appreciate it.

 

??? Possibly check operation of E-brake. make sure the levers are returning to there stops when released. I've never heard of this before. Good luck, and be CAREFUL driving.

 

e-brake works fine, i have some further testing to see if the e-brake will prime the system, but i am pretty sure it is the front brakes, ill let you know when i do.

 

i would bleed the brakes maybe???

 

As I am thinking about this some more, another thing that comes to mind is the check valve for the booster could be leaking. Put a vacuum pump on it, if you have one or borrow one. Apply full vacuum and let it sit, the gauge should not move much at all in the course of a few hours. If it bleeds down slowly, the check valve could be bad or the booster itself could be leaking. The lack of full vacuum in the booster will make the pedal feel ineffective. When you are "pumping" the brake, the intake is building back up some strong vacuum, and viola bakes are back.

 

if there were air in the system it would be constantly soft. it is not.

if the booster were leaking wouldnt it be soft anytime after a while? If i cruise at 40 this doesnt happen. ill check it though. where would i find a gauged vacuum pump?

 

Thats why I'm thinking maybe the check valve, because at lower speeds there is plenty of vacuum available to operate the booster. At higher speeds though (freeway+) there is very little vacuum available to the booster. This is why there is a check valve in the line, to hold a "strong" vacuum in the booster for your first initial push of the pedal. As you continue to push the pedal, the throttle plate is closed building stronger vacuum and continuing to help with brake assist. If it was soft all the time I'd think it was the booster leaking. I bought a vacuum tester at Advance Auto Parts a while ago I think it was like 20-30 dollars. I use it mostly just for bleeding brakes though. Good luck. Let me know what you find out i'm curious now to know whats wrong.