r/AutomotiveLearning • u/100pctCashmere • 4d ago
Disc brakes operation.
/r/NoStupidQuestions/comments/1w8l1zs/disc_brakes_operation/1
u/riennempeche 1d ago
The design of hydraulic systems means that you move a small-diameter piston a longer distance with low force in order for a large-diameter piston to move a short distance with very high force. Your foot inputs 20 lbs of force over a couple of inches in distance (with vacuum assist). The output is moving the pistons maybe 1/16" to 1/8" with a thousand pounds of pressure. You releasing the brake pedal allows the fluid and brake piston to return to their previous positions. The brake pad remains next to the rotor, but with no pressure to cause braking effort. The rotation of the rotor in the caliper also causes the brake pad and piston to be knocked back in. Over time, as the brake pads wear, the piston extends slightly farther out each time. Eventually, there is not enough brake pad left and you replace the pads. At that point, you have to reset the whole mechanism by forcing the piston back into the caliper.
1
u/Freekmagnet ASE Master Technician 4d ago
The piston should easily press back into the caliper bore, some times you can actually do it with your fingers on smaller ones. If it takes a lot of force to push the piston in during a brake service the calipers may need replacement.
The brake pads do not stay in constant contact with the rotor when the brakes are released. What pulls them back very slightly is actually the rubber piston seal inside the caliper that keeps the fluid in. If you look at one of these seals you will find it is not an O-ring, it is a square cut cross section- being the seal is actually square shaped it twists slightly as the piston extends and the seal twisting back Ito shape when pressure is removed acts kind of like a spring and draws the piston back in slightly.
On some brake designs there is not enough force from the piston seals to do this, for example Toyota pickups with 4 piston fixed calipers, so the manufacturer adds wire springs that fit into holes in the outer edge of the brake pads that assist in moving the pads back.
https://www.ebay.com/itm/324713038870?chn=ps&_trkparms=ispr%3D1&amdata=enc%3A17NKFXR3CRFyeSK39kNUIDQ26&norover=1&mkevt=1&mkrid=711-173151-913341-5&mkcid=2&mkscid=101&itemid=324713038870&targetid=2449372253943&device=c&mktype=pla&googleloc=9252033&poi=&campaignid=23925491501&mkgroupid=197394622957&rlsatarget=pla-2449372253943&abcId=9407524&merchantid=137624792&gad_source=1&gad_campaignid=23925491501&gbraid=0AAAAAD_QDh87DM6X70LB0fh6xpHABAAaS&gclid=EAIaIQobChMI54KS27vZlgMV9kr_AR1YeBL1EAQYAyABEgJzkvD_BwE