Brakes bleeding pressure at idle

[Pazuzu]

So, my brakes have been acting up lately. I have normal brake pressure when driving, stopping, etc, no apparent change there. But, when sitting at a light, my pedal starts to drop over 15, 20 seconds until I hit the floor. If I keep my foot steady, I lose braking and the car will start to roll. I can pump the pedal once and it goes back to full pressure, then starts to drop again. I can repeat this forever, but I've started using the e-brake at lights instead. I know that it's not something mechanical in the pedal linkage, I installed a very sensitive mechanical brake switch, and the brake lights remain lit when the pedal drops away, so it's not a piece of linkage bending or anything.

I am running a custom Bitz setup, so the vacuum is not stock, and I do NOT have the venturi in place (it's been gone for 3 years, never noticed a difference).

Hawk HKS pads, Zimmerman rotors, ATE blue, all changed 16 months ago. Steel braided lines changed out, oh, 7 years ago? New caliper piston seals installed 7 years ago. Daily driver.

Any ideas? Blown master cylinder? Just worn out fluid?

[jlex]

If you're not losing fluid, I'd say it's a master cylinder problem

[Pazuzu]

As far as I can tell I'm not losing any fluid. It's been this way now for 2, maybe 3 weeks.

[Pazuzu]

Well, searching (I know, shoulda done that first...) shows this is normal for worn M/Cs.

Yuck.
Between never researching upgrades, and John Walker's warnings about poorly built ATE replacements, and always being temped to get something bigger now in case I go with bigger brakes later...

Yuck.

[DRACO A5OG]

When you bled te brakes did you inadvertantly drop the pedal to the floor while pumping? If so that most likely cause the seals to get damaged.

Just make sure you bench bleed the new MC before install. it is messy so be prepared. while you have her apart, inspect the booster too for a leak

[Pazuzu]

Originally Posted by DRACO A5OG

When you bled te brakes did you inadvertantly drop the pedal to the floor while pumping? If so that most likely cause the seals to get damaged.

Just make sure you bench bleed the new MC before install. it is messy so be prepared. while you have her apart, inspect the booster too for a leak

I didn't change the fluid before this happened, but last time (14, 15 months ago) I used a Motive bleeder, no pumping.

I've done MCs before, and seem the damage that happens when you score the bore from over extending the piston.

[911pcars]

Primary master cylinder cup isn't sealing. Remove, disassemble and replace or replace the MC. Do not bleed by depressing brake pedal to floor.

Sherwood