Rear Brake Master Cylinder parts

Hello, I’m trying to bleed the system and I can’t make it...It seams that one of the rubber seals that go in the internal piston rod is defective, really I don’t understand how does it work? I made a picture to show how does it look like. It’s very common in all the china motorcycles, especially in dirt bikes, I think maybe someone can tell me if I can buy new rubber seals or a new pump if this is the problem. I tried to pump 100 times and no pressure; I cleaned and disassembled the entire cylinder but no results. Thanks.

Well it seems that nobody has been through this kind of issue yet....
today I tested the master cylynder and taking out the hose if I put a screw in the slot of the hose,the pressure of the pump starts to increase and it keeps there.So I must suppose that the pump is fine now.But when I attach the hose its really impossible to bleed the system.I guess I should try to change the hose and try to see what happens.

What happens to the fluid when you try to bleed? Is there any fluid that squirts or leaks out?

No,I mean thres no sound ,or bubbles coming out of the hose or caliper,I can see a very little movement at the pistons in the caliper,and its a strange movement because it moves 2mm lets say forward and after y realese the pedal it goes to the original position!!!

Maybe all the system is right and I´m not bleeding it correctly...

what do you think¿?

P.S.: one strange thing is that I can´t get any pressure,I mean could it be so much air in the caliper and hose so that after 100 pumps I can´t get pressure?

The caliper is supposed to return to a resting position after you squeeze... if not, then your wheel would always be locked!

Air can compress very much so if you have air in the line/system, you can pump 1,000,000 times and it will still not build pressure.

Are you using the bleed nipple on the caliper to bleed the brakes? If not, then you will never get the air out. You must use the bleed nipple on the caliper.

Bleed nipple will look like this:

http://static.summitracing.com/globa...s-639620_w.jpg

You need to attach a clear hose to the top of the nipple and something to catch the fluid that comes out. Then:

1) PUMP the lever to build pressure and HOLD the lever close to the bar to keep pressure. (Do NOT pull the lever all the way to the bar)

2) LOOSEN the bleed nipple for only a couple seconds. DO NOT remove the nipple... only loosen it a little (maybe 1/4 turn) until some fluid comes out. When fluid comes out, you will feel pressure release on the brake lever.

3) CLOSE the bleed nipple.

4) RELEASE the brake lever and pump again until you build pressure. This will put more fluid in the line.

5) REPEAT steps 1-4 until it's firm and you see no air bubbles coming out of the bleed nipple.

Other notes...

1) Do not pump and bleed so much that you loose all the fluid in your tank! If you do, you will only pump air into the line! Be sure the fluid is filled to a good level before you pump again to build pressure!

2) Sometimes air can get stuck in the joint between the hand pump (master cylinder) and the brake line. To get this out, pump to build pressure and SLOWLY loosen the brake line bolt. Be careful because fluid may squirt out very fast! If there air in this joint, you will probably hear a very little bubble and squirt sound when it comes out.


CC

I tried the steps you mention ,but the problem is that in step 1 I can´t get pressure to start bleeding .
i heard about using a syringe and a vacumm cleaner to take out the air inside the line,what do you think? In youtube I saw a few DIY videos ,but I think maybe its worst the remedy than the pain

I would not recommend a vacuum cleaner. It is still best to stick with steps 1-4 but first be sure all the connections/bolts are tight! It can take a very long time sometimes... just keep with 1-4 and read them carefully. If you are doing the rear brake, it is very easy for the fluid in the top cup to run empty. Be sure it is always full!

Some questions:

1) Did you remove or change any parts of the brake system?
2) Are you using the bleed nipple?
3) Can you give us some pictures of your setup?

CC

sure I can the rear brake pump is posted already in this thread,disassembled and the caliper I can take some pictures and post them.The flex hose too.

Do you need a picture of the whole setup in the bike?

and I´m using the bleeder nipple and this happened after I changed the rear brake pads.
The procedure I followed as always do for my beetle and motorcycles,I just loosen the bleeder nipple a bit,and then press the two pistons back,here I loose a bit of oil but its the only way to force them back(lot o pressure),now that I´m mentioning,I realize that this time they were really hard to push them back and I had to make a lot of force against the rotor in order to move them back,just in case this helps...

If you need more quality pictures I can manage to take them
thanks

txm,

If you have successfully bled brakes before then you know the bleeding process okay. If that's the case then perhaps:

1) There's a lot of air in the lines when you do a complete brake job and a lot of bleeding may be normal.
2) A mistake could have been made during the reassembly of the master cylinder.

Was there a problem with the brakes that made you take apart the system? Did they stop working?

CC

I disassembled the pump because I couldn´t bleed the system and I thought the pump was bad because I couldn´t get enough pressure,then I decided to test the pump.to acomplish this I put a screw in the threded hole thats intended to be for the hose,and after 2 or three pumps the pressure was great and whats more important it was constant ,i mean no pressure loss,what let me thought that the rubber seals inside the pump were fine,and the pump was correct.So now I believe that I must test the hose and the caliper,isolating them to continue troubleshooting.

The problem is that i tried to find a blind nut to prevent the oil getting out of the other side of the hose after i make pressure to see if it was a pressure loss but ,here in my country I can´t find one with the specifications for the screw of the hose(this could be confusing I know...)

The caliper seems to be very basic,I mean just a hole with a bifurcation to both pistons and another comunication to the bleed nipple.

but know I´ve another theory that I think could be the answer to the problem.Ithink the problem is basically in the design of the caliper,lets see what do you think.-...I´ll upload a little drawing tring to explain my point.

Basically i think theres a "air" cell that no matter how much oil you put in the caliper its always there unless you turn the caliper 90º to the left because of the position of the bleeder neeple.

I meassured the distance between the base of the hole and where the upper part of the pistons are and i found 10mm,that means that a huge volume of air could be there,agreeing with your theory that maybe theres a lot of air and no matter how many times I pump,I can compress all that air.

Could you follow me?
thanks

Well,today I could bleed the system to 90%,now its a bit spongy but I recovered the brake.
What I did to get pressure was
1)I took the caliper out from the rotor
2) I put beed neeple hose upper than the caliper upper than the hose than the master cylynder

basically I hanged all the system and used the gravity to bleed the air
I think my cavity theory was right and there was a "air cell" near the pistons,which I bleed following the method described above.

Hopefully tomorrow I´ll test the brake and see what happens

OK.It worked fine !!! and I had a nice off road mini trip today....all was fine !