Losing Braking Pressure, Internal Brake Master Cylinder Leak... I think Options

[Hazmat]

I need some advice on possibly a bad brake master cylinder? I'm onto something here I think!

I have noticed periodically when I apply consistent pressure while braking, the pedal will start to go soft and sink to the floor almost and I have to reapply pressure to avoid rolling forward/backwards. I have had all my brake lines checked for leaks and also had a brake fluid flush done and I'm still noticing it happening every now and then. I have also checked the firewall inside the car for leaks and underneath the hood only to find nothing out of the ordinary.

Now for my stupid question, the braking hydraulic system is it's own system away from the clutch slave & master in our cars correct? There is only a Master Brake Cylinder and nothing else?

Basically how I'm looking at it is

-Clutch Master & Slave Cylinders (It's own system)

-Brake Master Cylinder (No Slave Cylinder)?

I apologize in advance if I sound repeating or if I'm over thinking what I'm really trying to say. Just would like some input please & thank you!

This post has been edited by Hazmat: Nov 4, 2010 - 12:36 AM

[Spider77]

Typically "spongy" brake response means your getting air in the lines. Since you have not seen evidence of brake fluid under hood, I suggest you check you bleeders and see if they are tight.

This post has been edited by Spider77: Nov 3, 2010 - 6:34 PM

[Hazmat]

The bleeders are tight and I'm still seeing no signs of leakage inside and underneath both sides of the firewall.

Was reading and found this to be what I'm pretty confident what I'm experiencing:

"When constant pressure is maintained on the brake pedal (like when you keep your foot on it at a stop light), the brake pedal will begin to sink to the floor as the brake fluid leaks internally in the master cylinder."

I'm guessing I'm on the fast track to a new brake master cylinder?

This post has been edited by Hazmat: Nov 4, 2010 - 12:37 AM

[presure2]

yup, sounds like a bad master,(there is no "slave" in the brake system) or since you have an ST possibly bad wheel cylinders.
if your not seeing any leaks, its probably the master.

[blu94gt]

I just replaced a bad master cylinder on a friend's car that was doing the exact same thing. Seals in the master cylinder were bad so it just didn't hold pressure, but no fluid leaks so it can be misleading. He had been checking all his lines, calipers, etc thinking he was losing fluid somewhere.

[Hazmat]

Thanks for the help guys! I took it out again and I got it to do what it was doing again although it still does it periodically. It's enough for me to have already ordered a new toyota OEM replacement though.