Brake pedal sinks under steady pressure.

If I step on my brake pedal (2016 Sport Access Cab) it sinks slowly to the floor. I have a good pedal when I step on them ... but if I hold the pedal, say at a stop sign... it sinks.

I have replaced the rotors shoes and brake hoses... bled the system.... but still I have a sinking pedal.

I would think it was a booster as I have a firm solid pedal when the truck is off.

Toyota says.... (wait for it...) They all do that...

Anyone else have this happen??

 

Sounds like a brake system leak to me. Have you checked your hoses, lines, bleeders and calipers to make sure they're not leaking. A bad booster won't result in the pedal sinking (I had a bad booster on my 2000 XJ). The only way the pedal will sink is a lack of pressure in the system, meaning you're adding pressure with the pedal and it's escaping somewhere.

 

Master cylinder is suspect.

 

Sounds like an internal MC leak.

 

I agree. I thought that since the new ones are getting a master cylinder recall and this sounds like a master cylinder symptom, its very possible.

Also its very easy to see if "they all do that." Pick a random truck on the sales lot and ask for them to start it for you. You dont even have to leave the parking spot, you just want to sit there with your foot on the brake...

 

Same experience with 2018 Sport (under warranty) equipped with the vacuum brake booster. Took it to two dealerships while explaining a sealed system containing a non-compressible fluid should show no settling of the pedal unless there was a leak. "They all do that".

No, they don't.

Third dealership, I pointed out a trace of brake fluid trickling down the booster drum. They contacted corporate, gave me a loaner on the spot & replaced the MC.

 

i would go back to the other two dealers and have a nice but firm conversation

 

No fluid leak? Bad mc. No way it's okay. Safety issue if you are sitting behind someone holding the pedal steady and it stays releasing.

 

Yeah... you should be able to depress it several times before it firms up
You bled the master cylinder, brake lines and brake actuator?

 

When you bled the brakes, did you place something under the brake pedal to keep from fully depressing it?

You never want to press the brake pedal to the floor, as you can damage the seals in the master cylinder by over extending the cylinder.

 

all bled fine.. and is a firm pedal with no leak down with no engine running..
I think at an idle there's not enough vacuum...
I may plug the master cylinder and see if the pedal drops..
no leaks anywhere ....

 

I read this thread yesterday and today I paid closer attention to mine. At a stop light if I put a little more force once stopped the pedal will go down more. It won't sink if I keep the same pressure. When I pulled in my driveway I checked it again and the same way. When I shut the engine off I then released the pedal and pressed it again, and no sinking. With my foot on the brake holding the same pressure, I started it up and the sinking feeling was there, however, once released and pressed as long as I keep the same pressure it will not sink, I have to put more weight/pressure to get it to sink. I have a 2018 Sport double cab and I have had the Master Cyl replaced under the recall.