Brake Pedal Sinking

[Sawdusted]

Hey Guys,
Have a '03 Lexus RX300 exhibiting sinking brake pedal at stoplights primarily. Also while pedal is sinking, it would seemingly reduce pedal force and I can feel the pads loosening its hold on the rotor. Felt like classic case of bad master cylinder.

Ordered a brand new original Lexus master cylinder from a Toyota/Lexus stockist (they had it ordered from Japan) and replaced it last weekend. Fairly simple R&R, including bench bleeding and then bled brakes at each corner.

However sinking pedal symptoms still present. After braking and coming to a stop, I can tell that the pedal st stop position is definitely lower than it ought to be. When I lift off my foot and immediately push down again, it would rest at the higher position, then gradually sink.

Could this be a bad brake booster? Or some proportioning valve? I don't seem to be loosing any fluid and pads are at about 25% left which is still serviceable. Granted it took me about 2 months to get the replacement master cylinder, I hope other parts were not damaged due to the extended period it took me to R&R the master cylinder. I hope it is not a faulty master cylinder. It's brand new in box,

Any thoughts guys?
Thanks in advance.

 

[901Memphis]

Sounds like a leak somewhere. Could be slow enough to not leave a mess but enough to let pressure out.

 

[WobblyElvis]

If the car has ABS, I would look there first.
or
Perhaps you bought another bad master cylinder?

If it's not losing fluid, it has to be getting from the pressure side to the return side somehow.

 

[redbone3]

May be a special brake bleeding procedure for cars with ABS?

 

[OneEyeJack]

Put your foot on the brake pedal. Start your engine. The pedal should go down just a little. What your describing is probably a bad booster.

 

[spk2000]

Sounds like a leak to me too. I had that once which was very hard to diagnose and I would bleed it good for several days until enough air would get in and cause trouble. Look around connections for seepage and check to see if level is dropping any. Might not show much as it could be like a teaspoon amount.

 

[901Memphis]

I had a leak that was from rusty lines that did had the sane symptoms and the amount of fluid leaking was very minimal. Once I replaced all the brake lines it was fixed.

 

[ThirdeYe]

Originally Posted By: OneEyeJack
Put your foot on the brake pedal. Start your engine. The pedal should go down just a little. What your describing is probably a bad booster.


This. I had the same symptoms on my old Integra. Turned out to be the brake booster/master cylinder. The pedal would sink to the floor when trying to keep the foot on the brake when stopped.

 

[Ducked]

Originally Posted By: OneEyeJack
Put your foot on the brake pedal. Start your engine. The pedal should go down just a little. What your describing is probably a bad booster.


Why would a "bad booster" (faulty vacuum servo in British) cause your brake pedal to go to the floor? I'd have thought you'd loose servo assistance and your pedal would get harder.

If there are no leaks or air in the system, and the fluid isn't boiling, it sounds like a faulty master cylinder to me.

 

[Sawdusted]

Thank for the replies guys. Just came back from the mechanic. Bad news.
ABS valve body is leaking such that it is not holding the valve closed when ABS is not needed. Brake fluid is leaking past the valve, causing the brake pedal to sink. This would explain the symptoms. No drips or loss of fluid, brakes are not spongy.

Not sure if the ABS pump can be rebuilt, but it's gonna be a costly fix to the tune of $2K estimated.

Sigh

 

[kschachn]

Originally Posted By: Sawdusted
Not sure if the ABS pump can be rebuilt, but it's gonna be a costly fix to the tune of $2K estimated.

Yes they can be but you're going to have a hard time finding a reputable place. Maybe someone else can help with that.

I think the problem is liability, the device itself isn't that complicated but you really do need to test it after repair. I have a friend with five old BMWs and when one of his ABS pumps went out he tried to find a place to rebuild it. He ended up getting one at a junkyard and so far it is working OK. The price for a new one online was something like $2500.

 

[redbone3]

Salvage yard time.