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ajc1989

Gas Gas TXT 2010 Brake Issues

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I am having issues with my rear brake on my GasGas TXT 2010. The bike features the rear brake model with a separate reservoir and hose line through the rear swingarm. My rear brake was functioning (although not particularly well) prior to rebuilding the master cylinder. Since fitting a new piston, I have been unable to get any fluid to flow from the master cylinder to the rear caliper. I appreciate this topic has been covered in various forms previously and I have look through most threads to date. The situation has become very frustrating and I’m just looking for any advice which might assist! I have outlined all the attempted techniques/issues I’m encountering so far.

  • Rebuilt the master cylinder with a new seal kit. Piston appears to return to rest position fine. Master cylinder piston appears to function correctly. No significant friction when the piston is pushed down
  • System won’t push or pull fluid through. I have tried attaching a syringe directly to the master cylinder inlet port. This was met with resistance and I was unable to push any fluid from the inlet port through to the caliper bleed port.
  • I have also attempted to connect a syringe to the reservoir hose and push fluid through whilst activating the piston. Again, this was unsuccessful with no fluid pushed through.
  • I have fluid flow from the reservoir to the inlet port on the master cylinder. The inlet port was cleaned out upon rebuilding the caliper.
  • Unable to get any fluid to pump from the master cylinder to the rear caliper. Tried bleeding in a conventional way with an Allen key and 8mm spanner on the bleed port.
  • Attempted to use a syringe on the bleed port to create a vacuum and pull fluid up through the bleed port. Unable to pull any fluid up this way.

I am unsure if all of the above points are pointing towards a new master cylinder? Although the new piston appears to operate correctly in the master cylinder bore. Any help would be muchly appreciated to aid my frustration!

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Can you use the syringe to push fluid through from the bleed nipple.Put a few turns of plumbers PTFE tape on the nipple threads first

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5 minutes ago, huski said:

Can you use the syringe to push fluid through from the bleed nipple.Put a few turns of plumbers PTFE tape on the nipple threads first

Apologies, forgot to mention in the first reply. I have tried this technique - I put ptfe tape on the threads of the bleed nipple and a light smear of grease around the outside of the syringe hose to help provide an air tight seal. I was unable to push any fluid up the system. I wanted to try push fluid up to the reservoir, but couldn't push any fluid at all. Felt like there was pressure against the syringe preventing fluid movement up the hose. 

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have you tried disconnecting the caliper hose at the master cylinder and trying the syringe? It will at the very least help pinpoint were the problem lies. I suspect that either the caliper itself or the master cylinder may be the problem.

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On the basis that the brake worked(albeit poorly) before the master cylinder was worked on,Id be looking at the master cylinder.As above remove the brake hose from the master and see if fluid flows.I wonder if you've been given the wrong piston ,if you fit the old one  does it flow fluid?Least now we're not working it gives you something to do

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It could be the reservoir port hole is blocked.

Do you have some freeplay on the actuating rod? If the piston isn't being allowed to return all the way, then the reservoir port hole is either fully or partially blocked, and you won't be able to bleed it in a month of Sunday!

You must have some air gap between the piston and the actuating rod.

Same deal on front master cylinders. It is by far the most common problem we see in the workshop with trials bikes. So many people think the actuating pin somehow affect clutch or brake take up point, but it is not for that. It should be set so there is 1mm or so freeplay when in it's relaxed position. Please check that first before moving on. The actuating pin on the rear brake lever can be adjusted for length by undoing the jam nut (10mm open ended wrench) and screwing the rod into the brake lever. until some freeplay is achieved. This allows the piston to go all the way back in the piston port, and opens the bleed port from the reservoir into the system.

 

Brake Pedal.png

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Also as Huski mentioned there is two very slightly different piston sizes. The kits look identical but there is a difference. 

AJP Master cylinder used on Gas Gas bikes up to 2012 used a 9.48mm piston, Gas Gas Part number BT280222220

Then with the introduction of the new AJP unit construction master cylinder the piston size slimmed to 9.38mm Gas Gas Part number BT280222220/2

And then again when Braktec took over the AJP construction it reverted to the earlier 9.48mm piston size. Very confusing!

You can usually tell the smaller 9.38mm pistons as they are a raw aluminium finish. The bigger 9.48mm ones are coated and are usually a bronzy to brown finish.

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