New (2 me) 2000 LS400 - Brake issue.

[Spinnetti]

Picked up a "survivor" dang thing looks new and the owner did his best to take care of it, so its pretty, but needs lots of mechanical stuff. I got it mostly sorted but have one odd one. Brake rotors and pads are good but it takes lots of pedal effort to get the car to stop at all, and it stops long. I never hear the ABS pump run (and it didn't when I bled the rear brakes) and a panic stop does not engage ABS and it takes a long time to stop - the fronts start to squeal a bit but won't lock up and ABS doesn't kick in and I hear nothing from the rear. I completely flushed the brake fluid and replaced the worn out rear pads and its a little better, but still something obviously wrong. There's no leaks, and when I start it up, the abs/track lights come on as normal for a few seconds and go out as expected. I scanned for codes and found nothing. Given no errors but no abs, is it likely that my ABS controller/motor is shot or could a internally worn master cylinder just not be engaging the brakes enough? (but that wouldn't explain the initial soft then high effort, low effect pedal)...

Thoughts?

 

[P Bateman]

High effort and low braking power normally starts with the brake booster.

You should have large vacuum line running from the intake manifold to the booster.
Pull it off with the engine running - it should create a large intake leak.
If it doesn't, check the hose for obstructions or collapse.

Pending that, plug the hose and take the car for a drive. If it's no better, then the booster is likely shot.

 

[Spinnetti]

Hmm. thanks. The pedal sinks about half-way without much effort. It only firms up toward the bottom. The ABS pump never runs too. I shorted the pump connection per the manual and the ABS pump didn't run, so I'm thinking the pump or the control circuit in front of it. I ordered a used one and will test that next. The booster is hooked up properly. I can pull that and see if the effort goes up, so that is a good thing to check, thanks.

 

[P Bateman]

The ABS pump should only ever operate when brake lockup is detected. Under normal braking, it is switched off.

 

[Spinnetti]

The ABS pump should only ever operate when brake lockup is detected. Under normal braking, it is switched off.

It didn't run when I did a "panic" stop, and also didn't run when bled the brakes and also didn't run when I did the test procedure outlined in the repair manual, so I assume either the motor or controller died.. I should know for sure this weekend.

 

[Spinnetti]

This is such a simple thing but I'm still stumped. ABS pump runs using the test procedure, booster seems ok, and booster check valve is good. Pads and rotors are good. Fluid completely flushed with no air, and new master cyl. New master helped but takes a lot of pedal pressure to get ABS to engage, but it does engage now. Still feels "off" and takes too much leg pressure to stop hard but this is the first 2000 I've had. My other one is a 91 with a 98 GS400 hydraulic master, master stop bracket and 95' LS front brakes so hard to compare.

 

[blinkerfluid]

What kind of brake pads and rotors are you running?
If the above are good, with good brake lines and a new master cylinder - there are three things you need to test or replace or whatever

1 - make sure you have good engine vacuum
2 - once #1 is confirmed, make sure the booster is 100% - if not replace it
3 - after going thru the above, the brakes still not work good - replace the ABS unit

 

[Spinnetti]

Thanks... after all that, looks like the brake booster had just enough of a leak to not work right. I replaced it and it stops properly now, though the pedals seems soft and long travel. It doesn't bottom out, just feels mushy.. I'll give it another bleed. I wonder if there's another master cylinder with a bit bigger bore to shorten up the travel and give better feel?