hope disc brake bleed -> sign of a bad diaphragm?
i'm wondering if someone can help diagnose the source of the problem i'm having.
over the past week, i tore my bike down to the frame and have been replacing everything except the derailleurs and brakes. i decided to do a brake bleed (hope mono m4's); i've never done one before.
i made some mistakes but wound up figuring it out and getting into a rhythm. i'm certain i did it right, except i've got one issue: when i squeeze the front brake lever, a little brake fluid comes out of one corner of the reservoir (one of the corners without screws). the harder i squeeze, the more fluid comes out -- and if the lever remains squeezed, the fluid keeps trickling. it's not much, but it's enough to drip on the ground.
here's what i've checked:
1) reservoir bolts are tight.
2) the rear brake is working perfectly, so i know i'm capable of a proper bleed.
3) i repeated the whole bleed on the front, to eliminate a fluke chance of having done something wrong.
4) i swapped diaphragms from front to rear. no change: the front leaked, while the rear didn't.
5) i then swapped reservoir caps from front to rear. now diaphragm + cap from the front were on the rear lever. BOTH sides leaked (notably, from the same corner -- the near left corner if sitting on the bike.)
6) i swapped diaphragm + cap from rear back to front. the problem went away in the rear brake and returned in the left.
i did not try just a swap of caps.
what puzzles me is how both brakes leaked with a swap of caps. i'm also puzzled as to how a cap could go bad. i don't think i did anything wrong.
is it more likely to be the diaphragm? these are approximately 5 years old. if it's the diaphragm, why didn't the problem switch levers when i swapped diaphragm?
the diaphragms can only be installed one way, so i know i didn't screw that up.
i see them online for $8 or so (seems high), but i don't want to order + pay for rush shipping only to figure out it's a cap or reservoir issue. if it's the reservoir, does that mean i need a new lever?
Last edited by tetonrider2; 06-09-2009 at 10:34 PM.
Weird. But probably the diaphragm. Call Hope and see what they think.
When you bled the brakes, did you happen to fill them with the pistons out to some degree and now the leakage occurs when the lever returns and fluid is returned from the system into the reservoir?
JC makes a great point, be sure the pistons are completely pressed back before you put the cap on.
If you watch the bleed video on Hope's website, they recommend replacing the diaphragm because you could have some leakage.
I would check to be sure the system is not overfilled first, and be sure to roll the diaphragm as seen in the video.
"Three balls at once...who knew?" - Cotton McKnight
when i bled the brakes, i had pieces of wood and a screwdriver wedged in the caliper (pads removed).
also, i did roll the diaphragm.
i know i'm capable of doing it the right way as the rear brake works perfectly, and this problem is continuing to happen after multiple (re)bleeds on the front brake.
i appreciate the comments, though.
guess i'll give hope a call.
UPDATE: the problem's been solved. i'm not sure if this is true for other manufactuers' brakes, but i backed off the bolts a bit (the ones that hold the cap to the reservoir) and that solved the problem. i did a quick re-bleed for the hell of it and to make sure all the air bubbles were out, and performance is 100% with no leaks.
i had the idea because the hope diaphragms have an imprinted channel pattern on the top side (and a matching protrusion on the bottom). i thought that perhaps this imprinted pattern was designed to let any drop or 2 of fluid circulate without being squeezed out.
backing off a touch (the bolts got quite tight as last night i kept thinking "compress that rubber part!"), i though, would let that circulation happen.
not sure if that's what's going on, but it worked. i have no other explanation for it.
thanks for looking, and i hope this info helps someone in the future who is doing a brake bleed. not sure about other brakes, but the hopes seemed pretty simple to do -- and i hear the newer models are even easier.