hello. my ls400 '90 is loosing brake fluid. i need to add every week, i've checked for the wheels, the rotors, the lines and there's not any leaking point. and i dont apreciate any leak on any where. i don't know where else to check. what could it be? i need to pump the pedal for a good braking feeling because if don't the pedal goes to the floor

 

Check behind the carpet at the brake pedal. Sometimes the master cylinder leaks down the firewall.

 

You may also be losing it under pressure form one of the intermediate lines and not at each corner. Have to get under the car and trace the brake lines for any signs.

 

Brake fluid can also leak into the brake booster and then get sucked into the intake and burnt by the engine so look for unusual exhaust smoke or smells,the under car pipes are well hidden in protective trunking making it difficult to spot pipe leaks.
A common way of locating a leak on calipers is to clamp the flexible brake hose off on individual wheels then see if the brakes still need pumping,if you find the brake pedal travel is less when say the right rear hose is clamped it is a good sign the fault lies around that area.
Have you replaced any brake pads recently and pushed the pistons back without first loosening the bleed nipple as this can cause the master cylinder seals to turn/distort.

 

if it is in fact going into ur booster as steve said..it can and will ruin ur cats