A 2000 Toyota 4Runner was stopping fine. A mechanic informed me that the brake pads were worn down. After the pads were replaced and bled the pedal sinks lower than before and the car requires more pressure to stop. I can pump up the pedal after 3 pushes. I took it back and he has since re-bled the brakes but the pedal still sinks low. Any suggestions?
If it hardens when it pumps, you still have air in the lines or cylinder(s). He simply did not bleed the brakes totally.
By the way, the fact that the vehicle was stopping fine does not mean the pads weren’t worn down. I don’t see any reason to doubt the mechanic. I’m glad you mentioned it, however, because it’s always better to get too much information than too little.
This is a stretch . . .
I wonder if the pads were really bedded in well, before returning the car to the customer
I’ve run into a few vehicles that required driving several miles, and getting the brakes smoking hot, before they were bedded in well enough and had a good pedal
Perhaps the rotors should have also been machined/replaced at the same time
Does your 4Runner have rear drum brakes instead of discs? If so, worn or out of adjustment rear brake shoes can cause a low pedal and even mimic a failing master cylinder.
Yeah, I agree, it’s a stretch…
but a valid question.
t’s actually not a stretch as it’s something I’ve run across a number of times. The first time was many decades ago and I seem to vaguely remember it being a Toyota. The car owner had replaced rotors and pads on the front along with the master cylinder and still had a garbage pedal that would sink halfway to the floor before taking hold.
The rear brakes looked fine but needed adjustment. Adjusted the rear and the pedal firmed right up. A larger volume of fluid needed to activate wheel cylinders on worn or out of adjustment rear shoes as compared to the fluid required to move caliper pistons on pads. Odd, but true.
I don’t doubt the front needed new pads. It is just after this was done that the pedal sinks and you have to push harder to stop. He says he bled them twice. It has been several months since it was done and no change. I will get someone to adjust the rear brakes. I wonder if it could be the master cylinder?
Thanks
@aquatic11 I suggest you ask him HOW he bled the brakes
Diaphragm brake bleeder
Vacuum brake bleeder
The old 2 man method where one guy steps on the pedal, and the other guy cracks open the brake bleeder line
Out of those 3 methods, the last one is the one I personally least prefer, because it ties up 2 mechanics
not only that, but there’s theoretically the chance that doing so will actually damage the master cylinder seals, causing it to bypass. The piston travels much further than it normally would, causing it to snag . . . and tear . . . on some crud inside the cylinder, which it would normally not contact
I say “theoretically” because I’ve heard about it, and read about it countless times, but it’s never happened to me . . . yet . . . the few times that I do have to bleed brakes this way
When you’re stopped at a traffic light, with the foot on the brake pedal, does it slowly sink?
It’s a 15 yo truck. Don’t discount the possibility of a bad master cylinder. It just may be coincidental, but it sounds like its time.
Adjust the rear brakes and then the master cylinder.
Pump the pedal repeatedly to get it as high as possible and then hold continuous pressure on it for a minute or two. If the pedal sinks at all with no fluid leaking out anywhere the master cylinder is most likely failing as @BK mentioned. An aging and worn master cylinder will often fail when bleeding the brakes.
Once all the wheels are bled, the load sensing proportioning/by-pass valve at the rear of the vehicle must be bled.
Tester
Rod Knox
I suspect that “aging and worn master cylinder” might not have failed if using the diaphragm brake bleeder, or the vacuum brake bleeder
I’m speaking from personal experience here. We have some vehicles in our fleet that are even older than OP’s car, and many still with the original brake master. If at all possible I use the diaphragm brake bleeder, and so far I haven’t had a brake master fail soon after that brake job
It sort of sounds like the master cylinder is bad. This can happen when replacing pads, during the bleeding process, esp if it is done by pushing on the brake pedal. The MC piston extends into areas it usually doesn’t go into, and tiny burrs there can damage the seals. Before spending too much money on other possibilities, I’d be inclined to replace the MC and see if that firms things. Assuming the more simple of the ideas above like checking the rear shoe adjustment doesn’t work.
I think dB idea’s above about the pads not seating properly is a good one too. Whenever I replace the pads on my Corolla, or even the shoes on my Truck, I rough up the surface of both the pads and the discs (or drums) using some 120 grit al-o2 sandpaper. I expect you can google the correct pattern to do this.
Edit: Be sure to bench bleed the MC when you install a new one.
Excellent advice about bench bleeding the master cylinder
I’ve seen a few guys install a new brake master without bench bleeding
They never did get a good pedal
+1 for Tester. I was going to suggest this but he beat me to it.
there are some new things going on with the abs systems and the inlet valves and the solenoids for abs. we are not idiots who cant understand the simple mechanics of bleeding brakes in fact ive done my own brakes since age 16 im 37 now. the issue you are looking for is going to be in the ABS PUMP. the valves get any kinda corrosion from the brake fluid that no one ever needed to change before and suddenly everyone has a really good reason to flush it and no idea. whats going on is that corrosion get into the copper and well Im not gonna get all scientific cause I only know the means and not the science. but ABS valves are now a thing and they get wrecked by the corrosion
so now like your abs system could be not overriding correctly. but in all serious no matter how dumb I sound I know what im talking about and its not air in the lines. ABS system can now cause~ pulls to the right or left, in rare cases some cars can even be disabled from stopping suddenly do to a valve issue and it thinking it has to engage when it doesn’t and ur car is stopping like your on ice on a hot summer day…………, you can get not a SPONGY BUT LOW brake pedal. there is a difference. when its spongy and u aply pressure it feels spongy lol. when its low and not spongy and u aply pressure it will go down far but feel very firm see. if it goes down to far and feels FIRM that’s your abs system starting to brake down. if it goes down to far and feels spongy that’s air in your lines. if it goes down to far while u are at a red light but seems to work otherwise, your about to lose your bakes pull over and have the master cylinder replaced.
You have revived a 4 year old thread so the person should have solved the problem by know. Also profanity violates the forum rules so edit your post or you will be flagged.
I cannot over stress how important the comment by @Tester actually is… Please dont overlook this info.
As a side note…why on earth did the mechanic bleed anything? Unless he opened a line it was not needed at all.