99 Lumina LTZ Overheating/Losing Break pressure

[procrast]

Overheating: the following problem recently began: when idling or driving in stop-and-go traffic my temp gauge moves to the half-way mark. (it used to always stay near the low end of the gauge). As soon as I get back into highway driving the gauge returns to the normal low end.

Brakes: Recently my brakes started to lose pressure when I hold them down for a while (e.g. when waiting at a red light for a while the pedal begins to sink to the floor and the brakes start to fail) I brought it to my station and asked them to check them, but they told me there was nothing wrong (hoses were good, not kinked, fluid levels were good). They said that the rear brakes just need to be adjusted and the only way to do that is by using the emergency brake whenever I park the car. I havce tried this for the past few days and the problem is getting worse.

Any help/insight on either/both of the above issues would be greatly appreciated.

[maxwedge]

Brakes sound like master cyl going bad. If no leaks, check for coolant loss, get a scanner on it and see if the fans go on at 222 degrees. 3800 or 3100?

[jeffcoslacker]

My '97's temp goes slightly higher than half when driving in traffic without the A/C on. When the fan cycles on, it drops back to about 1/3.

With the A/C on, it sits rock steady at 1/3 (fans running constant)

If you are only going up to around 1/2, there's no problem.

Your master cylinder is "bypassing", they call it.

A seal that would normally hold fluid pressure when the brakes are applied is ruptured or worn out in the master cylinder, allowing pressurized fluid ahead of the piston to sneak around it when held down, causing pedal drop.

For some reason, the onset of colder weather always seems to make this suddenly start happening. My theory is that the seal is slightly less pliable at lower ambient temps, and can't seal as tight.

[richtazz]

I agree with Max and Jeff that you need a new master cylinder. If there are no signs of fluid leakage, then the master cylinder is the only thing that can cause a sinking pedal.