Brake Issue On 1997 Ls400

[Andrew D]

OK, so here’s today’s brain teaser that I am hoping one of you experienced mechanics will be able to help me solve.

A few months ago my trusty LS400 developed a soft (long travel) brake pedal. Pads and discs at the time looked ok, but I do remember the travel increased after a particularly hard breaking episode on the M5 due to some numbskull in a 5 series BMW (say no more).

Checked brake fluid and this was pretty black so changed and bled the system but without improvement. After about a week the ABS pump started to kick in every time I set off (within 10 yards) after starting the ignition. A slight rumble of the ABS pump with vibration in the pedal and then same level of travel on the pedal, this only ever happens once until you restart the engine. The pedal does come hard after pumping three times and then softens upon release. The ABS ECU has no recorded fault in its memory, the ABS light does not come on and all wheel sensors are reporting correctly. The ABS rumble only happens once after engine starts and the car moves, if the car remains stationary the ABS does not kick in. So I am thinking that this is something electro-mechanical that is linked to the wheel speed sensors which is causing the ABS pump to kick
in. The problem has not got any worse over the past 4 months and my driving style has adjusted to the long pedal, but I would really like to find a solution.

I am just about to change the front discs and pads as they are now a little warn. Whilst this is likely to reduce the pedal travel I do not think it will influence the overall issue.

Any ideas?

[Mybikes a cx500]

I don't know if it's related to your problem, but a relatively common issue is the caliper slides sticking, which you can tell as the pads will be unevenly worn (thinner at one end than the other )

That will certainly affect pedal travel.

[Andrew D]

Mybikes a cx500, thanks but the calipers are all sliding correctly and pad wear seems to be consistent with disc profile.

[Titch LS]

Andy, sounds to me like it could be connected to a worn brake master cylinder. You can test for this by blocking(clamping) the brake lines to all four wheels (a bit difficult I know), so that they are isolated from the master cylinder and ABS module. After clamping, try the footbrake pedal again and see if it is still unusually soft with long travel.. If it is becomes harder with less travel than before, the fault is with one of the wheels. If still softter than normal the fault is with the master cylinder or ABS module, or, air in the upper part of the system. Just hope and pray that it's not your ABS module that's at fault as they are horrendously expensive. If it's the master cylinder, the new seals are not that expensive to replace. Hope this helps.

[Andrew D]

Andy, sounds to me like it could be connected to a worn brake master cylinder. You can test for this by blocking(clamping) the brake lines to all four wheels (a bit difficult I know), so that they are isolated from the master cylinder and ABS module. After clamping, try the footbrake pedal again and see if it is still unusually soft with long travel.. If it is becomes harder with less travel than before, the fault is with one of the wheels. If still softter than normal the fault is with the master cylinder or ABS module, or, air in the upper part of the system. Just hope and pray that it's not your ABS module that's at fault as they are horrendously expensive. If it's the master cylinder, the new seals are not that expensive to replace. Hope this helps.

Thanks Titch I'll certainly give this a try next week when I can get the car up on stands. Greatly appreciate your input and truly hope it's not the ABS module.

[steve2006]

If it is the ABS modulator unit I have one off a 1997 LS400 in my shed.

[Andrew D]

If it is the ABS modulator unit I have one off a 1997 LS400 in my shed.

Thanks Steve, if it does turn out to be the ABS Module I will be back in contact. Greatly appreciate the support from this forum. BTW does anyone know where I can get hold of a Lexus LS400 Mechanics Guide?

[steve2006]

THere are 2 volumes plus an electrical supplement in official book form bu these fetch a couple of hundred pounds a set, if you take a look on eBay you can normally get the same on CD ROM for a few pounds.