Brake Pedal pumps up after long drive if brake is not used.

[ericthered]

On my '94 Q45t (195K miles) with TCS I have a brake problem that has appeared over time. My usual driving is harsh with curves and hills, and usually my brakes work fine.

However, if I drive far without touching the brake pedal, say 30 highway miles, the brake pedal will noticeably require further travel before feeling pressure and engaging the brakes. Recently I drove probably 100+ miles on open road without touching my brakes, and whoa, when I wanted them, they were way down near the floor (but they were still there without any extra pumping).

Every time the brakes immediately pump back up. If I lightly pump the brake after driving 30 miles, the next time I use them they feel fine. This happens most of the time, sometimes it seems OK (maybe I don't remember using the brake).

I think this problem has been there for a little while, but getting slightly worse. Now that my commute includes 30 miles on flat highway, I notice it more often.

About the car: Rear pads very good, front pads about nearing 50% wear. Brakes do not shimmy when pressed (i.e. I cannot sense any rotor warp).No leaks or loss of brake fluid over time. I have flushed the brake fluid in the past 3-4 years, but not recently - and done no hydraulic work on the brakes since I purchased her 5-6 years ago.

There is some small vibration in the front tires... they are due for a change, but I have driven with much more vibration without this phenomenon.I have not experimented to see if this is peak speed related, there seems to be a correlation, but that would be weird. I live in the San Jose area, so it doesn't get too hot or cold, and my car is always garaged.

And one other weird thing happened. A week ago I started my car to go home. After a few seconds the ABS light came on. I drove a bit. Shut the car off/on twice, each time the light came back on. I drove home fine with the ABS light on. Since then the ABS light has never come back on (except as a test pattern). The FSM suggests that would likely be an intermittent electrical problem, and most likely unrelated, but it is worth mentioning.

All the usual problem suspects don't seem to be indicated here. (pad wear, rotor warp, severe tire vibration, hydraulic leak, holding cylinder pressure). Maybe this has something to do with the power assist drum? Or something in the ABS system I don't know about? I don't know.

Without other clues, I may start by rebuilding my master cylinder, just for good measure.

Thanks for any ideas,

Eric
Modified by ericthered at 11:29 PM 11/29/2009

[Q45tech]

Replacing or rebuilding master cylinder every 10 years is good advice just as it [rebuilding] is with all four calipers and [replacing] the flex lines that feed them.

Nothing worse than a blown line, or gasket from lack of mainteance.

[Q451990]

You might also check the function of the brake booster check valve and the condition of the vacuum hoses feeding it.

Heath