weird inconsistent brake symptoms

[J__]

im running the 4runner calipers/300zx vented rotor fronts and 240sx rear calipers with 300zx rotors in the back. have the larger master cylinder, stainless lines in front but rubber ones in the back.

 

the brakes feel very solid and pedal through is hard as a rock and engages after about 1 inch. however after a long cruise on the highway (lets say maintaining throttle at 120km/h), the first time i step on the brakes the pedal through is extremely long and "spongy". feels just like when the brakes overheat from a hard lap around the track. however after a few pumps, the pedal goes back to its rock hard feel.

 

i dont think the brakes/fluid would over heat since no brakes were used during that 'cruise'. the only possibility i can think of is the heat from the headers is heating up the brake resevoir... i do have a heat shield there however.

 

what other things can cause this? anyone have the same symptoms? tia

[JMortensen]

There is something known as pad "knockback" which is described in detail here: http://www.stoptech.com/tech_info/wp_knockback.shtml

 

but to be honest I've never heard of anyone complain about it on a street car.

[cygnusx1]

The pads backing away from the rotors due to vibration could cause a little of this. Warped rotors?

 

EDIT: Jon beat me to the punch. Also wheel bearing play?

[J__]

i checked my wheel bearing and there is no play. it could be the warped rotor. because my previous rotors were warped pretty severely. however since i put in new rotors and retightened the wheel bearings, it still occurs but not to such great extent. maybe the new ones r uneven =( thnx got the info, i'll read over the page. is that the only possible cause that can cause such symptoms? thnx

[Pop N Wood]

Master cylinder