I just swapped out my brake pads on my 01 Ls430 this weekend with the Akebono pads. It was a fairly easy job to replace all pads, add anti squeal paste to back of pads and install. Once everything was completed I screwed back on the brake reservoir cap and started the engine. I pumped my braked pedal a few times for the brake pistons to set in. I noticed that my bake pedal has some brake tension which slowly decreases when my foot is still pressing down on the pedal. I did not notice any brake fluid leaks of any kind at the calipers or by the master cylinder. My brake fluid level is full and maybe higher now since the new pads install. Any idea what could be causing the brake pedal to slowly depress to the floor? I do not believe any air could have gotten in the system as my brake fluid reservoir was full when installing the new pads and also no signs of leaks at the calipers. I did a check on the front 2 without the new brake pads (old pads used for test) to see if the pistons were coming out and they both did. I did not try the rear ones yet without the pads as it was getting too dark. I read it could be the calipers need rebuilding or maybe the master cylinder. Anyone else had this problem?
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Did you open a bleeder at any time? It only takes a second and it has to be bled.
Did you take it for a test ride? Sometimes when you fully depress the pistons (especially all of them) it may take quite a few pumps to get the pistons fully set back where they belong. I've done a lot of brakes in my day and never had a soft pedal after just swapping pads.
All fluid levels are full and no signs of leak before and after the pad change out. I spoke to a mechanic and he said it is probably the master cylinder needs to be changed out and this is very common when changing out the pads and pushing in the brake pistons to fit the new pads in. He mentioned that when the brake pistons are pushed in, the junk and water that is accumulated in the system gets pushed backed into the master cylinder. After pumping the brakes with the new pads installed the junk somehow breaks a seals in the master cylinder and causes an internal leak within the master cylinder and not external. This will cause the pedal to go down to the floor. Bleeder was never opened at any time and the fluid reservoir was full so, I ruled out any air getting into my lines. I'll update this post after getting the master cylinder changed out.
After reading the post above I called my brother in law who is a certified mechanic to ask what he thinks. He said that its possible to damage the master using the common method of just prying the old pads back before removing them to allow for the new pad thickness. It has happened to him and he said that for years he has cracked open the bleeder at the caliper and depressed the pistons and closes the bleeder just before being fully depressed so no air gets in and so fluid doesn't get forced backward.
I guess it makes sense to be safe but I've changed many pads over the last 30 years and have never had a bad master because of it..
[QUOTE=DanozLS430;7188815]All fluid levels are full and no signs of leak before and after the pad change out. I spoke to a mechanic and he said it is probably the master cylinder needs to be changed out and this is very common when changing out the pads and pushing in the brake pistons to fit the new pads in.
I dont think it is common at all. If it was, we'd be replacing the master cylinder every time we change our pads. I've never had the master cylinder replaced in any car I have owned.
This is NOT master cylinder. I repeat, NOT master. Click on my nick and check my previous treads. I had this problem, struggled with it and had it fixed.
Master cyl's failure on Lexus is UNHEARD of !
Basically the problem is either disturbed dirt going back into system and/or caliper/pad misalignment.
Search my posts please.
My initial guess based purely on the symptoms would be there is air in the system (start up the car, hold the brake down, shut off the engine while continuing to hold down the brake pedal; count to 20. If the position holds, you are good.). Another possibly, as others mentioned, is the seals in the master aren't sealing. This is pretty unlikely for Aisin master cylinders, as I had one on my old Toyota truck that I replaced after 16 yrs with a brake system that had never been flushed. There was a bunch of rusty sludge at the bottom of the fluid reservoir, yet the seals still held. First thing I would try is to flush the entire brake system with a quart of fresh fluid. It's been a while, but I generally start from the DS rear, PS rear, PS front, then finish with the DS front. The general rule is to start with the wheel furthest (in the sense of brake line length) from the master cylinder. However, on my Honda, it specifically said in the FSM to start with the DS wheel, so go figure. Personally, I don't think it makes a difference.
I've done lots of brakes/pads and have never had your problem. But again, I flush the fluid like every 1.5 yrs.