Front Brake Rebuild issues

[dusty_viper]

Hi All,

On my 2010 GasGas, I have an XBlok monopiece caliper (just like this one... http://www.thehellteam.com/xblok-brake-caliper.html) Anyway, the slave piston seals were faulty so I rebuilt this with new seals and pistons to which 3 of the 4 pistons went in fine but the 4th one wont go no matter how hard I try. When i try to get the 4th one in, there's no problem until it hits the seal then it wont go no further. I know you normally have to put a little bit of force to get the piston in but even after applying a lot more than the other 3, I still can't get it in. I can feel the seal compress a little when i try but then it doesn't go past it.

After removing the seal and comparing it with the old one, I noticed that the old one was slightly smaller? I thought that if anything the old one should be slightly bigger due to the swelling over time. I was hoping someone could clarify this as I think I might have a dodgy seal which has been made too big.

Any help would be really appreciated.

Thanks a lot

Jake

[sirdabalot]

Compare it with the other new seals and swap into one of the other cylinders to see if the problem moves to that one. Do the seals have a tapered cross section, if so, is it fitted correctly. Is the piston correct. On assembly use brake fluid or specific rubber grease as lubrication. The above presumes the caliper is disconnected from the system, if not, open the bleed valve or remove the reservoir cap to allow fluid to return/escape, otherwise you'll be fighting back pressure. 

[2stroke4stroke]

Did you get absolutely all of any corrosion out from the seal groove?

[dusty_viper]

Hi all, 

Thanks for your replies, apologies that it's taken a while to reply - been flat out with work! I could compare it with the other seals but I don't want to take them out now that they are seated in. Unfortunately I don't have one of those piston removal tools to get the pistons out without damaging them so I don't want to risk damaging the new piston now they are in - I might have to get them out though. I didn't notice a tapered section but I will have another look. Yeah i was just using brake fluid as "lubrication" to pop them in. Caliper is all disconnected and the banjo bolt is out so there is no pressure. I will check the seal groove again to confirm that there is no more corrosion but I did check this and remove any that was in there. I will double check again.

To save having to pull the pistons out again, do you know whether the seals shrink with time? I would have thought it would have been the opposite and swell up over time.

Thanks

[drevil]

There is more than likely some crap left in the caliper. Get some of the following and really go round and clean it.

https://www.amazon.co.uk/Stainless-angled-degree-Antimagnetic-Tweezer/dp/B0722Y2XXM/ref=sr_1_9?ie=UTF8&qid=1496834465&sr=8-9&keywords=pcb+tweezers

I was surprised how much more crap came out than I thought.

When pushing pistons in you should need the lightest of pressure using your thumb.

 

 

[dusty_viper]

The plot thickens. I can't see no corrosion on the seal groove but it definitely looks like the seal isn't sitting in properly. I did run a knife round it to try and clear any corrosion which might be there but it looks all clean. I did try one of the other seals in the same part and this was also causing issues even though this was fine in another port so it's definitely something on the caliper.

I'm waiting for a tool to come to remove the yellow bore cap and then im gonna try get a dremel too inside with a nylon wheel on to make sure it's nice and clean so i'll keep you all updated. 

I'm starting to wonder whether i should just bit the bullet and buy a new caliper! Lol