I’ve got over 200,000 miles on her.
Seems like a couple months ago after driving a two weeks in substantial rain, I’ve developed a squeak when depressing the break.
It doesn’t happen all the time. One time when starting the car with my foot on the break, push the start button, as a vehicle is going through it start up procedure. The battery first , then the squeak, before the engine started while my foot is on the break.
It sounds like it’s coming from the engine side of the firewall.
Please advise

Blessings

 

When was the brake fluid last flushed? It could be related to the brake system powering up during the start cycle. Or everything is just wet after 2 weeks of rain and the squeak will go away when it dries out.
Brake fluid should be flushed every 2 or 3 years as it absorbs water which can cause internal corrosion of expensive brake system components. You have a lot of miles and things wear out.
You may also want to check for stored codes and inspect the condition of the serpentine belt and all related pulleys as it could be a pulley squeak just as it starts to move on start up.
Try to find a local independent Toyota or Japanese car specialists to check it out, or a hybrid specialist. Do not go to the dealer.
Good Luck.

 

Note; my first para here is not directed at the OP:

My personal observation is with a new vehicle, owners follow the service schedule during the warranty period and then they start slacking off.
So I keep reminding folks to open up the thin owner's manual and keep track of service items [at dealer or other shops or my favorite DIY.] Following or exceeding the suggested maintenance [which includes inspection] helps diagnose real issues.

Back to OPs question:
We need a bit more description to assist you diagnose remotely. The set of questions below are brake related and we need to go down a different tree if the noise is in the engine bay.

The Squeek you hear, is it from the inside cabin [brake pedal area]? Assume astronaut position and take a bright light to see play in pins/pedal.
Is it from the outside ... engine bay or the wheel.
Is the squeak when the wheel is in motion or stopped still. [if in motion coming to a stop or during stopped to moving].
Do you see rust on rotors?
Are the rotors smooth?
Does your vehicle suffers from drag? [when you let off the gas, does it coast well? Has your MPG dropped since the squeek]
Abnormal brake pad wear?
Does your center hub become hot to touch after driving.

Answer these questions and most likely you will be able to diagnose on your own.

One more fact about Lexus vehicles is that the OEM rotor develops surface rust [not deep rooted but surface spot] very quickly after a wash or rain. If you drive a bit and use brakes the rust spots get rubbed off. [When engine is not running [hybrid] you get to hear the noise a bit more and it takes a bit more of actual stops to rub off the rust spots].

Salim

 

How long does it take or a professional to perform a brake flush on a Lexus 2014 RX 450 h ?

Blessings

 

Around an hour. The charge varies by geographic location and shop, but is usually between $150 - $200.

 

These are standard labor hours from two different work cost calculation programs. Multiply them by the cost per standard labor hour at the shop.

 

For folks who want to learn about billing:

Auto industry works on term "book hours" for the type of work for vehicle/yr/model [Some tasks are easy on some vehicles and some take more time]. Behind the scene the establishment holds back some amount and then pays the mechanic yet another. If the mechanic is fast and has honed his/her technique (s)he can do the job in less time [makes more $/hr] and if not an expert puts in the extra time out of his own time (makes less $/hr). The establishment uses the difference as profit after deducting operating-cost/warranty etc.

System then rounds up the book hours in 1/2hr increments.

Then the number is multiplied by the labor rate/hr. The labor rates depends on geographical area and establishment. Currently they range from $80 - $250 with exotics it is upwards of 500.

It is a pretty complex system .. where the actual work contract is between the mechanic and the owner of the vehicle. If owner does not pay the mechanic legally can file a lien against the vehicle. Layers are built in where the owner of the vehicle is told before time the expected cost and work only proceeds after the permission is given. Often times verbal permission is needed to proceed with work as things are discovered.

Salim

 

A preliminary invoice is also issued, including a calculation of the cost of labor and parts.
As I mentioned earlier, the cost of replacing a particular part is taken from the program. For example, an oil change takes 0.7 man-hours. One man-hour at this shop costs $175. Therefore, the total cost is 0.7 x 175 = $122.5.
Often, a mechanic is paid an hourly rate (for example, $20), regardless of the type of work he does or how quickly he does it (of course, if he works very slowly, he will simply be fired). I worked as a mechanic in various places, and in 3 out of 4 places I was paid an hourly rate.

 

The shop flushed the brakes three times making sure there was no air in the system.
The squeak sound is the fluid pulsating through the lbrake line. You can feel it the exact same time you hear it.. the brake pads are good. The rotors are good. The calipers are good. What do we check next?

Blessings

 

If it is not the brake pedal, and the brake caliper slide pins are greased, and you have no codes indicating a problem with the expensive to replace brake force distribution module or ABS system, and no problems with the master brake cylinder and the brakes work, given that it is a 12 year old vehicle, I would live with the squeak.