how thick should my b pads be?

Iron_inc

Iron_inc

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Having some trouble with the rear brakes on my build. The piston moves in and out but as soon as the piston/pad gets to the rotor the master cylinder gets hard and moves no more.almost as if in coming up short am I missing something? or are my brakes pads to worn? There close to 1/4 inch on each side
 
Yes, the piston puts little to no pressure on the rotor but the mc wont pump anymore fluid, unless I manually push the brake piston back in with a screw driver. I know The mc is good
 
And it's not soft when you press the lever? Sounds like you need to bleed them by what's happening physically. But the way you describe it, it doesn't. Deep you try to bleed them again?
 
If you haven't done so, you probably need to disassemble the caliper and clean it out. Sounds like the piston is sticking. It should pump all the way out, actually coming right out of the caliper if there wasn't a disc there to stop it.
 
Jake it pumps normally untill it reaches the rotor (maybe just a coincidence) then it gets stiff as hell so I k ow the mc works and its pressurized

5twins you might be right it was seized but I soaked it and broke it free, maybe a lil more rust???? The piston has a throw of about 1/4 to 1/2 does that sound right or am I coming up short?
 
By "throw", do you mean the piston will come out only about 1/4" or 1/2" then stop? If so, then it's sticking. Like I said, if nothing was stopping it (the disc), the piston should pump right out of the caliper. You say it was seized and you broke it free but did you actually remove it and clean the inside of the caliper? That's what usually needs to be done.
 
Ok I agree you should clean out your mc. When you said you knew it was good I assumed in was brand new. Hopefully that works.
 
Well, the MC probably needs a good cleaning too but I'm referring to the caliper.
 
Thanks fellas I didn't realize the piston came all the way out I will bet on it snap. If the piston is seized is there a way to get it out? Besides blowing my mc. I know one member on here was thinking of making a tool
 
I blow them out using compressed air. With the caliper removed and the line detached, blow air into the line hole. You'll need to have the bleed nipple installed and tight so all the air stays in and forces the piston out. Also, you'll need a good seal around the blow gun into the line hole. I use the tapered rubber tip on my blow gun and just push it into the hole really hard so it seals. One more thing - wad rags around the piston because it's going to come out with a "bang" when it lets loose. No kidding, it will be loud enough to scare you sometimes. And if you don't block the piston, it can shoot right across the room. So, needless to say, don't have it pointing at you and keep your fingers out of the way, lol.

If the piston is really stuck, it may only come out part way and stop. If this happens, force it all the way back in using a big C clamp and hit it with the air again. You may have to "work" it like this for a while before it will come all the way out.
 
A couple of things.

1. Did you oil the piston? That's a no-no. Oil ruins the seals in the brake system.

2. Do you understand the piston comes out through the square caliper o-ring and then stops at the rotor? It is the elastic nature of the caliper o-ring that pulls the piston back and keeps the pads right at the rotor, ready to clamp.

Tom
 
A caliper is as simple as things can get. There are not many parts, The body, a piston, the square section o-ring, a dust sheild And the bleeder.
The stock piston is a chromed steel part. It can rust, If it was frozen, it is rusted bad. It probably needs replacement. Mike's sells a stainles steel replacement.
The o-ring might be ok but the dust sheild is probably shot. The rebuild kits aren't to much.
The body being aluminum won't be rusty but the groove the o-ring sets in gets full of crud, binding the piston. The crud in the groove and the rust on the piston are probably what's stopping the piston.
Leo
 
Tomterrific said:
A couple of things.

1. Did you oil the piston? That's a no-no. Oil ruins the seals in the brake system.

2. Do you understand the piston comes out through the square caliper o-ring and then stops at the rotor? It is the elastic nature of the caliper o-ring that pulls the piston back and keeps the pads right at the rotor, ready to clamp.

Tom
Click to expand...

No I did not oil the piston ,if the seals were bad I would either lose pressure (and leak brake fluid) or the piston would not move at all.
Yes,I understand how the caliper and piston work ,yes the piston is supposed to come up to the rotor and stop but its supposed to put pressure. On the rotor (which is what stops the bike) I think what your not understanding is the piston is stopping at or closeto the rotor with out putting pressure on it.
 
5twins I will pull the caliper ASAP and hook the ac up to make sure I'm getting full throw thanks for the help
 
Yes, if the piston is only coming out so far then stopping, it's sticking. These are old bikes. The brake system, both MC and caliper, usually needs a dis-assembly and thorough cleaning. When I first got mine home, I pushed it across the shop floor and put the front brake on to stop. It stopped alright, and stayed that way, lol. The piston wouldn't retract and the brake stayed on. I tried a thorough bleeding and flushing with fresh fluid but no dice. The components needed to come apart and be cleaned. They were all gunked up inside. In particular, be sure to clean that o-ring seal groove out that Leo mentioned. It gets full of dried, crusty brake fluid over time. That forces the seal out too much and that binds the piston.

All I did was clean everything. I replaced no seals or parts. The brake has been working fine and leak-free for going on 8 years now. I did need a new line though. The original rubber line had gone soft, giving me a soft lever feel no matter how much bleeding I did. A new stainless line fixed that.
 
I guess I didn't write that very kindly, sorry Iron. I never know who I'm chatting with and I'm not a good writer.

I had about the same experience as 5t with my XS when I got it. I used warm water, alcohol and finally brake fluid to get the parts clean and conditioned. I looked at the square sectioned o-ring and saw how one corner was worn a bit round. I flipped the o-ring so the unworn side made it like new. All new hoses because they were scarey.

Tom
 
No worries tom I still appreciate your time and effort. I have some liquid wrench (silicone based I believe) would that harm the o-ring or is warm water and alcohol my nest option for the piston?
 

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