201D; or so (Fig 3).
2. Have a helper push the brake pedal down SLOWLY until the indicator moves 0.020 or so then hold it until the reading on the dial indicator stabilizes. (Fig 4)
3. Zero the indicator. (Fig 5)
4. Release the brake pedal.
5. Now read the relative movement indicated. (Fig 6)
How much did piston retract? It better be at least 0.002” MORE than the manufacturer published lateral runout specification! The 0.002” is based on personal observation after measuring many vehicles, so your mileage may vary. But you got the idea. So, if the piston retracts 0.006”, your spec is probably going to be 0.004” giving you 0.002” clearance when the brake is released.
And, this checks the piston for sticking at the same time as it can’t retract properly if it is sticking in its bore. Piston retraction can also be measured at various intervals throughout the pistons travel. (I have seen several of them stick in only one area.) This works on multiple piston calipers as well due to the fact that it is NOT the hydraulic system, but the piston SEAL that is doing the retracting. So as long as you can depress the brake pedal and get the piston to move the 0.020” or so you need, if working properly it should still retract regardless of the action of the other pistons. So you may have to block the other piston(s) to get the piston you’re checking to move but it still works.
Although it really doesn’t take much time once you get the hang of it, I do not check this on every disc brake job that comes through the door. I’ve found it invaluable however for problem vehicles or those with high mileage and/or calipers too expensive to replace simply on a hunch.
JAMES A. BLAIR
• ASE Master Certified
• ASE L1 Certified
• ASE P2 Certified
• ASE Master Engine Machinist Certified
From technician, service and parts management, contract trainer, technical representative to shop owner/operator, Blair has worked nearly every facet of the automotive service industry since those part-time jobs at service stations during high-school. Blair has been a full time automotive instructor since 2000. In August 2012, he accepted the lead instructor position at New River Community and Technical College in Ghent, WV, and was tasked with developing both a new automotive and diesel program.