New brake pad install problem

[Pete Lenzini]

Hey Guys,

I've read tons of posts on rebuilding calipers and related subjects but don't think I've seen my question addressed. I'm finishing up my brake job which has included rebuild of all 4 calipers, new soft hoses at each corner, new front rotors, and am now trying to install new pads everywhere. I'm on the left side and can insert the outer pads on both wheels but cannot get the inner ones in. I can compress the pistons using a piece wood to pry or a pair of large channel-lock pliers. Before trying to compress the pistons, I lack about 1/16" of clearance to get them in. The old pads I'm replacing are not very worn, probably not more than 1/8" thinner than the new ones and the old ones slide in and out easily. My problem is that when I compress the pistons the small amount I need to get the new pads in by the time I pull out the wood pry or lay down the pliers, the pistons slowly creep back in to their original positions before I can insert the new pads.

What is going on? How can I keep the pistons compressed enough to get the new pads in? Is it the "self-adjusting" mechanism or the square O-rings at work? Is there residual pressure forcing the pistons back in? If it's a pressure problem would opening the bleeder calve help?

Thanks for your comments!
Pete

[Vic Skirmants]

If you have read the posts on rebuilding disc brake calipers, you must have run across "exercising" the pistons. If you do that, you should not have a problem inserting the new pads.

[Pete Lenzini]

Yes, Vic I've read your post on exercising the pistons. I've gone back and forth multiple times with the pistons but maybe I haven't let them come in far enough? My "old" pads are barely 1/8" thinner than new so if I expand the pistons with the old pads in, the pistons are only moving in a little more than 1/8" or so. Should I use a thinner piece of wood in place of the old pad so I can get the pistons to move in more than an eighth of an inch or so?

Thanks,
Pete

[Wes Bender]

Push the pistons in farther. They only rebound a certain amount. You haven't exceeded the amount they rebound yet. When you're eventually successful, you'll need to pump the pedal a couple of times at each application until the seals bed in. Otherwise, put the worn ones back in and use Vic's method. Either way works, Vic's is just quicker (and a tad more elegant).

[Pete Lenzini]

Thanks Wes. I think I've just about got them bottomed with the tools I have. I'll try to get them in further, but I think the problem is I haven't let them come out far enough so I'll also try pumping the pedal with the old pads in or use something thinner than the old pads.

Pete

[Vic Skirmants]

Yes, use something thinner. As Wes stated, you have to move the pistons appreciably to get everything to flex into place.

[Thomas Sottile]

Why are you changing the pads with little ware

[Pete Lenzini]

Age

[Pete Lenzini]

Thanks Vic and Wes for your advice. I got all the pistons exercised and all the new pads fit in easily. My problem was I wasn't allowing them to travel in far enough and not compressing them back far enough. I solved these problems by using a couple of pieces of steel I had laying around that were about the thickness of the brake pad backing plate or roughly 1/4" thinner than the old pads I had been using. And with some effort, I got them compressed back to where they were almost flush with the caliper. The biggest problem I had was pushing them back, but using large pliers, C-camp, and even an old ball joint separator I got them all compressed. I was able to develop a feel for when they were moving and bottoming. Now just have to bleed the system which I will try with my power bleeder.

Thanks for your help guys.

Petet