Hydraulic brake problems
 

I bought a used cannondale bad boy ultra with drop bars and 10 speed shifter levers and hydraulic brakes.

The brakes use a cable to hydro conversion master cylinder that is held to the steering tube with something similar to a front reflector bracket.

I bought the bike knowing that it had the bad a rear brake system and that the rear brake dragged. The seller had already sold the bike and had pulled out from the sale when he noticed the issue. The original buyer wanted nothing to do with it after knowing about the brakes and the guy offered it to me for $400 less (full retail of the entire front and rear kit)

My guess was that some used mineral oil.

Hoping (but doubting) a complete bleed would solve the problem I bled the brakes according to a guide on this forum.

Before the brakes where dragging and did not engage at all besides the dragging.

Now when I press the lever down the is a decent amount of stopping power for a split second. I could be spinning the wheel with the bicycle suspended as fast as possible and it was instantly stop the wheel but then right away the wheel can move again.

If on the bike a feel a jerk and then no resistance.

Visually you see the brakes pads squeeze and then slowly retract.

My guess is I need to rebuild the rear caliper...

I have not found a guide to rebuild the master cylinder but am hoping it is fine.

I am wondering if before doing this I should swap the front and rear tubes that go to the master cylinder rebleed and then try again... My thinking is if the rear suddenly works and the front does not then it is the master cylinder not the caliper that is damaged.

Also one thing to note...

I smelled the fluid that came out... It had a strong pungent smell.

My guess is that any DOT fluid would have no odor or smell sweet. The 5.1 I replaced it with smells nothing like the fluid that was removed. It actually had no discernible smell.
Or is it possible that the fluid had an animal detractor that smelled bad...

Anyways

What's the recommendation with this.

I have a mechanical and technical background with lots of motorsports experience but am new current tech bicycles.

Also are there other hydraulic calipers that are "mostly" compatible... Possibly not knowing what they are getting into a shop quoted me $25 in labor plus the rebuild kit.

Or I can do it myself (watched a video it's pretty straightforward but everything is tiny)

If I could find a used but working or new caliper for under $75 I would just do that

Thanks

too add to things... Is it possible to run a small lighter rotor with less leverage. What parts would I need... With the majority of braking done up front I would love to save some weight in the bike while making it so that about the same amount of force on both levers makes for a decent forward balanced braking.

Symptom fits with damaged master piston seal. Holds pressure briefly then loses it as fluid flows back past the piston into the reservoir. Try swapping the lines to confirm.

- joel

+1, if this were a car, master cylinder would be suspect, so that's where I'd start.

In this particular case, I think I'd put some decent (road) mechanical discs on the bike and get away from the hydraulic w/converter system. I'm a big fan of hydraulic discs on mountain bikes, but other than maybe the newest drop bar/hydraulic systems from Shimano, I'm not sure there's anything on the market yet in drop bar hydraulic systems that's not chock full of problems. Just an opinion.

Have not had time to check but if it turns out to be the master and not the caliper itself I will not be re-using it.

as of right now I bought a used Avid BB5 at the bike shop for $12 and slapped it on. A little overpriced just for a caliper but what can you expect for no wait and local. $20 plus shipping would have got me an entire pair plus rotors and in black but oh well.

If I can't figure out how to get cross top bar interrupts to work with the hydro system it's going to get scrapped anyways.

right now I installed a interrupt on the top bar for the rear brake and I love it.

My wife has been on vacation all week so I have had no time to play around. Once she goes back to work I'll have some free time to play with this stuff.

going to see if it's the master cylinder or not and then going to see if I can integrate an interrupt lever into the system. I know theoretically it should work but because of the location of the master cylinder cable routing is rather difficult.

thanks for re-affirming that testing the master cylinder wouldn't be a waste of time.

also the fact that there is no leaking or loss of fluid makes me think it is the master cylinder as well.

whats the best cable pull caliper out there? Want the front one to feel really good. Not to impressed with the BB5 but would rather have something cheap and affordable on the rear.

My opinion: Avid's mechanical disc offerings, as well as Shimano's, Hayes', etc will all offer pretty good, functional braking, if set up right. If you want really good, and I mean REALLY good, disc brake performance with drop bar levers, you can't get it cheaply, and even then, it's an iffy area at the moment. The road/CX market's just not there yet. Affordable, high quality and trouble-free hydraulic systems will be the eventual answer, just like it has been with mountain bikes.

Again, just an opinion.

So,
It took a long time for the new seals to come from the UK but they came.

Not even a hint as to instructions but I figured it out. The largest seal in the master cylinder was completely torn and damaged. It isn't a simple o-ring type seal it's more like a soft rubber plunger/boot that has to go around a shaft. Certainly an achilles heal, although hope claims of NO reported failures prior to mine.

Not a common product though...

I will say the feel is better than the BB5 road brakes so until something affordable exists I am going to keep these.

Hopefully with the SRAM and Shimano kits now out that affordable discs will be available within 2-3 years.

Until then Im going to stick with the hope