Normally, that symptom would be caused by overheating of the rotor. Rain would likely cool the rotor. Therefore, it would appear that the rain is causing the pads to slip. Unusual. But if the rotors are sufficiently glazed, that may happen. If the front pads are original, I question why the rear were replaced since they normally last 2x as long as front. That might support the glazed rotor theory since that would cause the rear pads to work harder when dry.
The first thing I would do is to replace the front pads and deglaze the rotors.

 

Thanks for the reply. The rear pads and rotors were replaced when I purchased the vehicle because the drivers side rear pad was completely worn to the rotor grinding. When i replaced them I cleaned the slide pins and lubed with sil glyde. The piston seemed to push in ok with the tool.

 

Double check torque spec on your wheels. Re-torque.

 

Hey there,
The rain-specific vibration during braking is a bit unusual. Since your rear brakes and rotors are relatively new, it's worth checking the condition of the front brakes and rotors. Sometimes, uneven wear or surface rust can cause such Weight Lift issues. If the front brakes and rotors haven't been inspected or replaced in a while, it might be a good idea to have them checked out by a mechanic. They can pinpoint the cause and recommend the necessary steps to address it. Safe driving!

 

We just bought a 2019 hilander xle and just experienced the vibration/grinding when braking during rainy conditions. We just took our car to have the brakes and rotors inspected and all is good with them. So why is this happening only when it's rainy when you apply the brakes?

 

I don't know about vibration, but the "grinding" can happen if your car sits out in a rain storm or very humid conditions. In those cases rusting of the rotors begins almost instantly. It's just surface rust and wont do any harm but it will sound awful on the first few brake applications. It wears off fast.

 

Have the same issue on 2018 Platinum Limited. At 30k there was pulsation in the brake pedal. Had front and rear rotors and pads replaced by Toyota dealer. In heavy rain rain we get the front end vibration shutter when applying brakes. Especially noticeable when applying brakes going downhill In the rain. Tough to trouble shot with dealer. They have to diagnose in the rain.

 

Thanks for all the feedback. With all the information everyone has provided I’ve got a couple ideas. I’m starting to wonder if I took the brake pad bedding process a little too far when the rear brakes were installed and inadvertently glazed the front brakes causing the shudder in the rain. I’m debating on just going ahead and ordering new pads and rotors for the front from Rock Auto. Also, I need to double check the rear brakes for glazing as well and try to verify if the right rear caliper is sticking or seized. If I remember correctly the pad on the piston side was worn completely down with a good amount of pad material still left on the outside. When I compressed the caliper piston with the tool it seemed to be a little hard to move at first but with a little effort went back in. I feel a little better knowing some others have experienced a similar issue without many answers from the dealer.

 

Here is a little update. Drove the vehicle most of the weekend with no issues but it was sunny all weekend. Today I inspected the passenger rear brake since that seemed to be the source of the worn rear brakes to begin with. I did two separate test drives of about 15 minutes and used a laser thermometer to get temperatures. Both rear brake rotors were within 20-40 degrees of each other from what I could tell. Jacked up the rear. Both tires spun relatively easy with just one hand. Took off the wheel and caliper on the passenger side to inspect the pads. They looked ok to me. Slide pins seemed to be moving fine. Although the bottom one seemed like it may be stuck at first it moved freely after a couple wiggles. Caliper piston seemed to be ok from what I could tell. It expanded when pedal was pressed and retracted when pushed in with tool. Checked torque of all wheels. I did not have time to look at my front brakes today. I attached a picture of the passenger rear pads.

 

Ive had this problem for years it seems with my 17 Limited Highlander. Everytime it rains, and it doesnt go away until its dry out. I also think Ive ended up replacing the rotors too many times. The dealer mechanic said Im probably hard on the brakes. (Im not). There is definitely some sort of water flaw with them- but no one knows what/why? Glad its not just me.

 

I had vibration under light braking. I replaced the rotors with aftermarket high carbon content rotors after researching the OEM rotors seem to be junk and now it's smooth as silk.

 

I plan to put new pads and rotors on the front at some point just haven’t gotten around to it. I’ll update if that made the problem go away. I drove it the other day when it was raining all day. The entire trip to town there was no issue. It wasn’t until I got into traffic with stoplights and everything it started to do it again. Then on the way home all was fine. It was strange.

 

We've been having this issue off and on in the rain since we bought our 2019 Highlander two years ago. Thanks all for sharing your thoughts!

 

We have noticed the shudder/growling when brakes are wet also. New tires and a new front axle (unrelated) so we know the rest of the front end is in good condition. Sounds and feels scary to be honest.

 

Anti-lock brakes engaging??