Hope mone M4 bleed problem.

struck

Hello,

My Mono M4 rear brake has completely lost power and the lever broke.

I bought rebuild kit for lever, and as its about 4/5 years old i bought new seals and pistons for the caliper and a Goodridge braided hose. Even got a Hope bleed kit, it works..but its not hard to do without..

Anyway, thorough clean and complete rebuild of caliper and lever plus new hose i thought would make it work almost as new.
It hasn't, lever has to be pulled almost right back to the bars to get very little power there is, and the most confusing bit and my main concern is why fluid is spraying out of the hole at the top of the master cylinder when i pull the lever?!

I can't find any info relating to this or the function of that hole anywhere.. have i missed something, is this diagnostic of a dead brake or something ridiculously simple like worn MC seal, too tight bolts, etc..?

I am 99% positive it has been rebuild correctly - spent all evenings last week slowly going over and over some decent detailed instructions on the web (built my two bikes from scratch and everything that goes with it, so im quite confident i know what im doing...... )

instructions i used are all on this website
http://steveukmtb.wordpress.com/hope-caliper-overhaul-minitrialsm4v2m6/

sorry for long post, wanted to give u as much info as possible.

also, its not so urgent as i have ordered a replacement Hope Tech M4, just if i get it working it'll be good upgrade for a mate.

Mancunianfightingcat

the little hole is to let air in above the diaphragm (the wavy rubber bit just under the top cap).. As the pads wear the diaphragm pulls down in to the fluid to keep the brake feeling the same.

It sounds like your diaphragm has split, or you haven't put it back in.

Mancunianfightingcat

part number 21 on here


http://www.hopegb.com/voir_mini06_image__tgrandeimage_1.html

struck

ok thanks, so it works in part with the metal plate at the bottom of the reservoir to compensate for pad wear?

I am positive the diaphragm is in there, so it could be split. though i feel its a bit optimistic that such a power loss is due to MC diaphragm? i bought a new brake thinking the bores that the pistons sit in may be worn, therefore the caliper was completely knackered. will have another proper look after fitted new Tech M4... had enough of brakes and fluid for a while.

would like to point out the same size pistons in the new design, why didn't they do that before? would have saved so much effort in production and tool costs etc surely cant be that much of a leap in engineering ideas/methods.

thanks again

Mancunianfightingcat

If the diaphragm is split the fluid will take the path of least resistance, which will certainly be through the split leaving little or no power to push 4 pistons.

fatblokefromwarwick

I have the Tech M4's & if you level up the reservoir on the bars and take the lid off and press the lever it puts/squirts no fluid out at all, and it's not meant to either.

Try a new rubber seal or clean it up, refill and replace carefully without overtightening the torx as they're only there to hold that cap on and not hold any pressure within.

Mancunianfightingcat

Come to think of it neither does my Mini Pros, you can take the top caps off and the brakes still function properly.


Sounds like there might be a blockage or something meaning the fluid is being forced to find a new route, like past the diaphragm....

struck

sounds like it could be that. will order a new diaphragm.

what do you mean exactly 'with top caps off'?

....MC top cap removed but with diaphragm in place? and brake should still function?

Mancunianfightingcat

top cap off and diaphragm out, as long as the calliper stays level it still works fully!

struck

you know of any areas in the system prone to accumulates and blockages? as i really gave the whole thing a thorough clean, plus new hose.

thanks for your help btw, although if i asked last week i may have saved quite a lot of money. nevermind.

topsey_turvey

Have you fitted a new Tech caliper onto old style levers? Are they compatible?

struck

um no i haven't. quite clearly says i rebuilt old mono m4, didnt work, then bought new tech m4.

have no idea if they are compatible, they probably are not.

hucking_fell

If you follow the Hope website's how to and roll the diaphragm onto the resevoir then there will be a little fluid on the upper edges of the resevoir which will then become trapped between the diaphragm and the top cap. This will 'bubble' out of the breather hole which is normal and will not effect the performance of the brake at all. After a ride you might notice tiny drops of fluid on the outside of the resevoir by the lever.

Mine are like this and work fine. It shouldn't squirt out though.

struck

yeh it sprays out, just small drops on the bar but everytime i pull the lever theres no power at all. open the cylinder and fluid level is really low.. as i see fluid 'spraying' out of top cap there might be chance of air in system so i go through whole bleeding process etc and making sure all pistons are moving equally and in sync, just to have spongey lever, no power, and more fluid spraying out of top cap again.

this is why i thought to replace seals and pistons in the caliper, and buy a new braided hose, but still nothing! so could be as simple as a stubborn blockage or split in diaphragm. i'll get the bleed kit out tomorrow and see if that helps force it through.

Mancunianfightingcat

Topsey Turvey wrote:

Have you fitted a new Tech caliper onto old style levers? Are they compatible?

Yeah they are compatible. I have a tech 4 calliper on my mini mono pro lever. Works fine, even the new tech pro uses the older lighter lever

nicklouse

if there is no power then there are 3 possibilities.

1 there is still air in the system

2 the master cylinder seals are worn/damaged/incorrectly fitted.

3 the piston seals are worn/damaged/incorrectly fitted.

a Split diaphragm will have no effect on the performance.