Today was new pad day at my house, but things got frustrating when I somehow introduced air into the lines just by pushing the pistons back in. (Recent new brake lines meant, otherwise, there would have been no need to bleed today.)
The googles says this can happen, but what I want to know is HOW do I do it so this doesn't happen next time.
Have you bedded in the new pads? I can't see how replacing pads would introduce air into the system, unless there's some other details you didn't share.
Plus 2 on ChasH comment. Some cars (Fiat 124 as example) always have a crappy pedal until the pads are bedded in. When I ran a shop we always bedded those cars in before returning it to the customer.
As an alternative, you can crack the bleeder screw open as you push the pads back in and close it just before they bottom out. It's like a mini bleed and cleans out some of the dirty fluid in your calipers. I'm not sure how pushing the pads back in would introduce air unless there was another problem already but it suppose it's possible. Almost anything is in the car repair world.
In reply to spandak :
If pushing the piston in puts air in the lines, they already have a leak.
I did a pad/rotor swap on my '12 Rav 4. Pedal travel was normal before the brake replacement. It had a decent pedal but would very slowly sink to the floor after the replacement. Made you feel like you were always chasing the ideal braking force, by just a little bit. Driveable, but not right. No line was ever opened, and no leaks were present. I bled the calipers anyway, and no change. I figured I must have forced some type of debris into the master cylinder, so with a new master cylinder in hand, I decided to try one more fix on the old one. I disconnected the lines and "bench bled" the master on the car. Reconnected and bled a stupid amount of fluid through the system. Pedal became normal after that. I can't explain why exactly. The fluid was clean and I never found debris.
My future procedure will include always opening the bleeder when compressing the caliper piston back in. good to get rid of that old fluid anyway, thus never forcing fluid backwards through the master. Then just a quick bleed should take care of any air.
TGMF said:My future procedure will include always opening the bleeder when compressing the caliper piston back in. good to get rid of that old fluid anyway, thus never forcing fluid backwards through the master. Then just a quick bleed should take care of any air.
If it's a car with ABS, you really don't want to push what's typically the dirtiest fluid in the system back up into the ABS HCU. Too many little passages and valves that need to be able to seal shut.
Norm
My old RAM was the same way - terrible pedal after brake work, would get better after a day or two of stop and go driving.
NormPeterson said:TGMF said:My future procedure will include always opening the bleeder when compressing the caliper piston back in. good to get rid of that old fluid anyway, thus never forcing fluid backwards through the master. Then just a quick bleed should take care of any air.
If it's a car with ABS, you really don't want to push what's typically the dirtiest fluid in the system back up into the ABS HCU. Too many little passages and valves that need to be able to seal shut.
Norm
there is iron in those words
In reply to ChasH :
Bedded in via a handful of easy slow-downs from 35-40 mph to a crawl, then a dozen or so 60-to-15 stops. Other than spraying the backs of the pads with Sticky Nasty Blue Anti-Squeal E36 M3, I just pushed the pistons back in and installed the new pads.
Are we taking about rear pads by chance?
What vehicle?
Old pad material on discs preventing new pads bedding in properly?
I forget if you changed rotors too. I'm guessing the pads are sitting on the ridges of the old rotors and flexing when you push the pedal.
My 2015 Silverado is doing that now.... old rotors with rusty ridges... flexes the pads until they make great contact.
In reply to wvumtnbkr :
Hmm, that may be it: I did not change the rotors. Thx 
('89 Saab 900, I changed the front pads only; car has no ABS, run-of-the-mill ATE calipers [not the weird old ones with integrated e-brake]; I'm easily able to lock up the wheels with the new pads.)
I had a similar thing happen after replacing front pads in my 175k mile Rx8. The pedal was soft, I thought it would get better once they bedded in, tried bleeding the brakes repeatedly but they never got better. Eventually I found the sliding pins on the calipers were frozen/corroded causing the pads only to contact the rotor on the piston side. After I freed up these the pedal was back to normal and braking great. Naturally installed rebuilt calipers soon after.
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