Brake Booster Problems?

[lsrxlex]

Hi all,

My brake booster effect decreases after 1 second of depressing brake pedal, at speeds over 45 mph, specially when I tried to stop at speed over 55 mph (depressing pedal real hard).

I have replaced master cylinder, front brake calipers, front rotors and pads, but to no help.

I already tested the booster as recommended by Lexus, and the test seemed fine.

What would be the problem? Do I have to replace/repair booster or any vacuum valves? and where to look for those valves?

(My car: 1990 ls400, 280k miles)

Thanks for your help.

[IS400]

I reconnected and soldered the trunk wiring that was broken (black wire) in the area of the drivers side hinge. It was way up in the loom and I had to take off a lot of black tape type factory wire wrapping and a few sections of flexible routing to get to the section that was broken (almost to the bulk head).

I was there to fix a non functioning trunk release and ended up getting SOLID braking as a bonus. I'd replaced a lot of brake parts and done two brake jobs trying to get the car to brake like my old ES300, but the ground wire in the trunk was what finally fixed my brakes! Hope it works for you!

[steve2006]

I assume you have done the test where you start the engine,turn the engine off then pump the brake pedal until it goes very firm then keeping you foot on the brake pedal restart the engine whereupon the brake pedal should go down slightly as the vacuum builds up.

If you are losing vacuum I would 1st check the servo vacuum hose and it's connections to the servo and inlet manifold, the non return valve and finally suspect the servo. ( I also think the servo may have a filter on it )

This is on the assumption that the rest of the braking system is OK.