Having a bit of a problem with my rear hope tech 4. After a bleed (following the instructional video on hope's website) the rear brake will only last about 4/5 rides max without the lever becoming really spongy. Thought I had fixed it a while ago after bleeding and replacing the seals in the caliper body - but it has happened again. The front brake is absolutely fine.
My first thoughts are fluid loss or air getting in - but where could this be happening?
Any suggestions?
Martyn
You still have air in the system...you need to spend more time bleeding it as the air is likely to be in very small pockets rather than a large single bubble - quite possible stuck round the pistons so it works until it heats up then it goes funny.
One thing you could try - put the bike front wheel high (so the rear brake is lowest point of the rear brake) - lock the back brake on and leave it on overnight...what should happen is any air will over the course of the night move to the highest point of the system (or the highest point it can access - so tilt the bike to the right slightly as that then means the brake hose is highest point of the caliper and air should hopefully move to there).
In the morning, release the brake lever and give it a feel - should be much firmer. something else to then do is pop the reservoir cap off and top up the fluid level - the air should have replace some of the fluid in there so topping it up should then keep everything sweet.
Might want to try it a few nights on the trot to see if you get a consistent feel.
This works most times but isn't always a permanent fix, but it certainly does help shift some (almost all perhaps) air that could still be trapped in the system - but I'm not 100% sure why.
If you do this early enough in the day/evening - give the brake hose a flick as you go past it, this will help shift any air that is already in the hose...
I wonder if you could tie an electric toothbrush to the calliper as it is bled to "shake" the bubbles a bit and help them release?
(okay, I've obviously gone mental...)
Rachel
I doubt it is residual bubbles of air as the performance would not deteriorate if it was.
Air under the diaphragm getting into the system or failed master cylinder seals would be my guess
Does it bleed through ok, or is there air in the fluid when you pump it?
Did the piston seals install absolutely ok?
I rebuilt my M4's back along, and inadvertently just nipped a piston seal as it went in - all looked fine, but the brake lever had loads of travel, and only really worked after pumping it, so it all came apart again, and that's when I found the problem.
The piston was barely 'moist', so it was only seeping a tiny amount of fluid, but that little split was enough to pull a tiny it of air back in with the piston movement.
I assume all of the hose unions are dry?
New copper washers maybe, if the old ones are a bit too 'set'?
Going to bleed them AGAIN tomorrow and see if any bubbles there. Getting to be a bit annoying now though to be honest. Also going to go and clean/dry everything now and then inspect tomorrow morning to see if any small leaks anywhere. If cant get it sorted this time then I think it may be off to a shop/get in contact with hope and see what they say
Slight hijack sorry...
I've got a seal kit for my older M4 brake, can anyone talk me through how to change the master cylinder seals please? Couldn't see any vid clips for that on the hope site.
TIA
You need to pump the pistons out on the side that doesnt have the bore caps first. So, keep on pumping the lever while holding the 2 pistons on the bore cap side in. The pistons on the other side will pop out. You then need some bore cap tools : http://www.chainreactioncycles.com/Models.aspx?PartnerID=79&ModelID=5244
for the older M4s I believe there are two different size bore caps to you need to get both size tools. I don't know if there are any make-shift tools you could use to get the bore caps off....though i'm sure someone will be along to tell you a cheaper way to do it.
Once the bore caps are off you can get the pistons out and then its simply a matter of replacing the seals and rebuilding them.
When I spoke to Hope they recommended using silicone lubricant for the seals and pistons.
HTH
Thanks for info, I was mainly looking for help with the master cylinder seals in the mini lever if poss?
Teh master cylinder is fairly straightforward. From memory ( its years ago I did mine)
I used the diagramme form the hope website.
Remover the lever - pivot bolt and unwind the reach adjuster.
Pull the rubber dust cover off the pushrod. (labelled adjuster screw inteh diagramme) Undo the circlip that holds the push rod in and withdraw pushrod Piston should then just come out. Teh seals are hard to get off the piston - IIRC I cut them off. Put new seals in hot water to soften them for reassembly. Lube everything with brake fluid, seals onto piston (make sure teh right way round) lube again with brake fluid, put piston in cyclinder and reassemble.
Its straightforward and easy Just be systematic make sure everythig is very clean and lube it all up with brake foluid
Great TJ many thanks