Hi All, I have other threads on brake related issues but I have a general question. I don't have a caliper that is sticking (at least not that I know of) but I am aware when a caliper sticks that the vehicle could pull to one side. If a caliper piston were to hypothetically get stuck, would the brake pedal actually return? It seems like the pedal would stay stuck unless enough pulling force is generated to allow air/fluid to enter past the cup/seal to fill the gap that would be created. I imagine the cup seals would seal in both directions of pedal travel.
I had a caliper stick once while I was on the interstate and in a panic stop I spun the car,the pedal didn't stay down. The culprit was the rubber hose had deteriorated and the fluid had softened the inside to the point that it collapsed. HRP
That's scary. So the hose probably acted as a 1 way check valve and kept pressure to the caliper past the failure point? If the hose had deteriorated and was soft, it seems the pedal returning would be accomplished by the hose deforming a little?
Retracting caliper pistons or wheel cylinder pistons, has nothing to do with "pedal return". Stuck caliper/wheel cyl pistons have no effect on "pedal return". Some very old cars have a pedal push rod that it mechanically attached to the M/C piston; so if that type had a sticking M/C piston, that could cause the pedal to not return. But even on those cars, a stuck wheel piston will not cause the pedal to "not return".
If the pedal didn't return under the "stuck caliper" (or wheel cylinder, which is what I think you mean?) condition, then it would be pretty hard to bleed the brakes?
Ok, there are several ways that a caliper can stick. First is an internal hose flap, collapse, obstruction. That it will prevent fluid from passing- either into or back out of the caliper. If fluid is blocked from going in, the opposite side will lock up or it will pull under braking to the opposite side. If its blocked from leaving the side that's obstructed wont release. Second is the pistons inside the caliper can get corroded and not retract. If they are REALLY bad they wont apply under pressure. Pistons that don't retract cause a pull to that side while driving and that side will be noticeably hotter than the other. Third is the caliper slides get sticky and the friction doesn't let the caliper body move back. This keeps usually the outer pad in contact with the rotor. Again more wear on that pad and more heat on that side. 4th is not really the caliper but the pad slides get tight from rust building on the casting under the stainless clips. The pads drag on the rotors. Sometimes it creates a pull while braking to the opposite side, and a pull to the effected side while driving. A pedal that doesn't return can be caused by friction against the pedal arm or its pivot. A bad bushing, cooked grease, obstructions, dragging the carpet, wiring harness, broken return spring. Some of those can cause increased effort in pushing the pedal. Some of these situations cause pulls that come and go or swap sides. It will pull right on one stop then the next stop pull right or no pull. From here and the other thread I'm getting the idea that you think the returning of the brake fluid is what pushes the pedal back. That is not how the pedal should return. While bleeding the pedal needs to return far enough that it lets the piston inside the master return to its stop. That piston is spring loaded inside the master but the spring is easily overcome.
Thanks, Sorry I was forgetting that the push rod can return regardless of what the M/C is doing since most are not mechanically linked together (this is assuming an external return spring for the pedal). What confuses me is if a caliper were to stick (say from rust , etc) but the hoses were fine. As the internal M/C piston/seal assembly pushes back from the force of the internal spring, fluid would normally flow back but the caliper being stuck and the M/C piston/seal returning would increase the length between the two. Assuming the original fluid was still forward of the piston/seal, a gap would be created due to the increase in length. Where would the air (or fluid for that matter) come from to fill that gap?
Thanks this is really great information to better understand brake caliper systems. I'm aware there is an internal spring in the M/C but you are supposed to have an external return spring dedicated to the pedal. I have mine set up with a dedicated external spring.
Take apart a master cylinder, see if you can figure out how it works. Might be that there is a way for fluid to get where it needs to go.
Thanks! I had seen some images online of master cylinder cross sections (someone cut one down the middle). What I don't see is how fluid/air can enter to make up that gap when a Caliper piston stays stuck. You would have to overcome the seal to allow air or fluid to fill the gap that is created when the M/C internal piston pulls back (if it indeed can). I wonder if the internal seals can block fluid in one direction but allow fluid to flow the other direction which would allow this. Here is a cross sectional view of an M/C:
It works on PFM The fluid intake and return port isn't labeled in your pic but its there. So is the equalization ports. Once the spring pushes to the intake port any sucking or pressure drop within the lines is over. Rear drum wheel cylinders can and will suck air if the pressure inside the brake lines is below the atmospheric pressure. That's why all drum brakes need to have a residual pressure valve.
I think my daily has a caliper sticking on the sliders, it clunks down there sometimes under braking like it takes some brake force to move the thing. I haven't taken the time to look at it.
Thanks it's really cool to learn about this stuff. Let's say you push the brake pedal in and you have disc brakes on circuit 2. At this point the rear seal is to the right of the intake/return port and pressure can't escape. The distance between the rear seal and front seal decreases and fluid pushes the caliper piston in. Now the caliper piston stays locked (because of rust, etc). The rear seal is to the right of the intake/return port. How would the rear seal move back enough to the left to the intake/return port such that the pressure can equalize? First a gap would be created unless the distance between the front seal and rear seal stays the same after the caliper locks up and they slide together enough that the rear seal reaches the intake/return port.
So you are stuck at reconciling a volumetric difference and pressure equalization on return. The large reservoir is Under atmospheric pressure, think of that air pressure as plunger, 14ish psi. on the large bore. In reality that's quite a bit oomph. On return of the spring loaded MC piston without the return of a caliper the pressure inside the brake lines will be below atmospheric. (That's on the wide side of the tapered seals.). The oomph pushes fluid past the narrow side of the seal and its easy because of the taper and low pressure on the wide side. The tapered seals seal better when pressure increases on the wide side. The pressure forces the seal wider. With a pressure decrease on the wide side, the wide side collapses.
Thanks so much for helping to understand this. I also had noticed the tapered seal geometry in the diagrams and cross sectional views of M/Cs I had seen online. I wondered and was about to ask if the seals were able to act like one way check valves with the pressure differentials and your explanation confirms that.
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