Brakes intermittently not engaging

[xdarkxfirex]

Have a 2008 Yaris. Bled brakes and changed fluid, replaced master cylinder, checked lines for leaks and rubber hoses. Master cylinder holds pressure. Sometimes I feel like I have to press on the brakes twice for them to engage. Someone said it could be my front rotors having some warpage even though pad life is still around 40%

[tmontague]

If the rotors are warped (very hard to do and rare) or if there is extra pad material on the rotors it can push the pads further back into the caliper which would make you have to pump the brakes before they engage fully.

I would check your rotors and/or swap on new rotors and see if it feels any different

[dogsridewith]

might make/confirm rear wheel cylinder adjustment has shoes close to drums (multiple reports here, including mine, that rear automatic adjusters didn't), re-do the bleed (how was this done? there are several methods.) and make sure front caliper bodies are sliding freely on the pins.
pedal freeplay? (need some so master cylinder fills. too much uses up pedal travel)

[xdarkxfirex]

Any time I replaced a master cylinder on this car, I bled it at the two front lines. About two weeks ago I bled the brakes with a neighbour. We did the front then rear. I found some air pockets in the rear. Hoses all looked fine. One had dirtier fluid than the rest. Also greased sliders and hardware. I've done a lot of hard braking with this car and they are crappy lightweight rotors. Sometimes even when I try to stop I feel the pulsating, then the pedal sinks a little. Other times I can press on the brake almost halfway, feel nothing really, and then again, and it engages. It doesnt sink, like a leak or classic bad master cylinder does.

Some people mentioned a bad booster can cause this as well, but I am doubtful. Boosters with leaks give a hard pedal. That or maybe a proportioning valve can have a problem. IDK im stumped.

[dogsridewith]

Stuck caliper sliders exacerbate pulsation because pads on both sides can't follow the rotor without bending it and/or flexing the whole caliper mount structure. (I know this, because I've spent too much time freeing up stuck sliders on Tercel and Corolla...rather than replacing calipers.)

Pads on suck-slider cylinders wear to a taper profile because the mount structure bends during brake application.

When the brake pedal is released, both the wobbly rotor AND the sprung-back-away-from-the-rotor distorted pads create extra freeplay that, in effect, has to be filled with brake fluid before the brakes start to do much.

Typical bleeding instruction is to start at the wheel farthest from the master cylinder and proceed closer.

[xdarkxfirex]

The sliders was fine, and pads were wearing pretty evenly. Move in and out the bracket easily.