YODAONE

On 1999 LS400 (and 1990 - 2000) a rubber hose extends from the union atop the intake manifold to the check valve on the brake vacuum booster cannister.located behind the master cylinder.

Before changing motor and transmission mounts I noticed this hose would slip 1/2" on the manifold union...caused by excessive engine movement due to engine mount failure and hardening of the hose baking atop the intake...

After replacing engine mounts this problem was not as pronounced yet still noticed a gap on the steel line before the hose stop from time to time..

The line was also rock hard.

Upon removal and inspection I observed.the hose end at the check valve side had visible cracks on the end...time to replace the rubber hose and hose clamps. The top hose running from intake union to brake vacuum booster check valve (shown after replacement)
This image also presents that hose configuration coupled with excess engine movement can cause this hose to loosen on the manifold union side...or even pull out completely(!)


This image depicts cracking on end of the hose connecting to the check valve on brake vacuum booster. (!!!) After 18 years, both ends of hose were rock hard and turned easily on fittings...The hose clamps had lost some tension and were also replaced.
This image depicts Lexus exploded parts diagram, Lexus P.N.'s and parts replaced. (Old parts shown)

The grommet between check valve and brake booster was replaced...still reasonably pliable...not brittle like PCV valve grommets but did replace anyway. The OEM used high quality rubber..

dicer

There is still a brake system if the power booster is disabled for any reason. Though application is a bit harder kinda like if the power steering goes out.
A much bigger safety problem is the rubber hoses that feed the calipers, if they break then the fluid leaks out. They also get old and cracked.

Losiracer2

Did you notice any weird symptoms up until the point of replacement or notice any restored performance in brake pedal feel/engagement after replacement?

YODAONE

There was no noticeable symptoms or improvement...but I am proactive on repairs, especially where safety is concerned.

The hose was hard and is cracked as shown which suggests replacement....

Toyota used good quality rubber but engine movement, age, heat, and climate took their toll on this hose.

Suggest all LS400 owners inspect and, or replace.

Don't forget to use OEM replacement hose clamps.

RA40

iTrader: (6)

Thanks for the reminder. This is a hose I've not replaced yet. How hard had the grommet for the check valve become? (Thinking it would be hard like the grommet on the PCV valve that had to be cut out for removal.)

YODAONE

The brake vacuum booster check valve grommet was not in bad condition...but less supple than new grommet....

It was replaced because it was inexpensive and was unsure what its condition before extracting.

I scored across the exposed face of the original grommet and with a twist/pull removed it easily with a pliers.

Scraape

nice. just replaced this on my 98 as well as other vacuum lines.
just put a slightly smaller hose on there so it doesnt have a chance to wiggle off or any of that.

YODAONE

Does Lexus offer "a slightly smaller hose"??

These are metric fittings on metric hose with compression clamps (not screw clamps!!) engineered for this particular application; high heat, engine movement, vacuum cycling (does not collapse on suction)

Not encouraging or advocating anyone use aftermarket anything, except OEM here...

Last...for safetys' sake, be sure your engine mounts are in good shape...if seperated then excessive engine rocking movement can csuse the brake house depicted in this posting to separate from the manifold line.. (I have witnessed this movement)

An earlier response suggested you possess one final braking action (vacuum assisted) provided the check valve and/ or grommet seal integrity are not disrupted.

No one anticipates having to use two feet to stop a car.