Leaking Maguras

The HS33 on my KH29 is leaking. It stopped having any power, and I bled it to see if that was the problem. But now there’s oil all over the rim, the caliper, and even up the crossover cable (it’s on the caliper with only one hose attached).

So, do you think the oil is leaking from the crossover hose, since there’s oil up there? Are there any common trouble spots, in terms of leaking, that I should check first? I’m thinking that if it’s a leaky hose, I’ll probably just buy a braided steel cable set and upgrade the puppy.

Let me know if you have experience with a similar problem, and how you solved it! Thanks!

I’ve had leaking problems in the past. To find where it is, you need to do the equivalent of submerging a tire to find the leak. Do a bleed on it so the system is both full and fully-capped (closed system). Then do a quick surface clean of everything using a cloth with some grease-breaking detergent (like Dawn). Dry thoroughly. Then pull the lever to increase the pressure in the system, and look for where the leak is coming from. Once you know where it’s happening, then it’s a different problem-solving exercise to figure out how to fix it. With any luck it will be a simple part replacement.

There might be better approaches, but that’s how I went at it. Key is to get the oil cleaned off so when you pull the lever you can find out where the oil is coming from.

Im sorry I cant be of much help. When my next to new Maguras lost pressure, I stopped by my LBS and asked them what was up. In my case there was oil leaking onto the lever. The owner of the bike shop called up Magura, and they sent a bleed kit and whole new brake lever and top cylinder setup(dont remember what it is actually called) all without charge. I paid a few dollars for the bike shop to do the fix.

I had a case of a leaky Magura that was caused by one of the bolts actually starting to crack from being overtightened. The head of the bolt was actually starting to come unattached. Replacing the bolt fixed the problem.

A rather worrying problem because if the head of the bolt had actually broken off it would have required some minor surgery to remove the broken bolt.

It’s not an oil

I’m not sure what brake fluid is made of, but it’s not oil. There are several different types, DOT 1, 3, 4 and 5 I think. These aren’t compatible with each other, so you must know what type of fluid to use before you add any.

I rebuild my motorcycle calipers every few years, as crud and corrosion makes them stick and leak.

There is a rubber ring around the brake piston that seals in the fluid. If you get any oil products, like wd40, onto this rubber ring, it will swell up. The brake will then leak. Be very sure never to get any oil products near your brakes. There is a special synthetic brake grease you can buy to lube the pins and piston during assembly.

I don’t know why yours are leaking. Rebuilding a caliper is the sort of thing you want to do every so often anyway.

It’s pretty simple, there are few parts. After removing the pads, use the pressure of the fluid to drive the piston out. If that won’t work, use compressed air. Clean the parts with soap, water, and fine emery paper. If the rubber parts look new, you can reuse them, although a new kit isn’t usually very expensive.

The Magura rim brakes use a mineral oil. They do not use DOT brake fluid. DOT brake fluid is nasty stuff and will mess up paint and do other nasty things. Some bicycle disc brakes do use DOT brake fluid.

I followed Tom Blackwood’s good advice (thanks, Tom!), and found the leak on the crossover cable’s coupling where it went into the slave cylinder. I just took it off, cleaned threads etc., reattached, and then bled it. Seems to be holding up now, but I’ve ordered a braided cable set anyway–the plastic hoses really have no flexibility and are a bit of a pain to work with.

I used standard Magura Blood fluid, but have used sewing machine oil in the past (my last Craigslist set of HS33s came with a pint of the official Magura stuff). Many trials riders just use water because it makes bleeding the system really simple–they submerge the whole thing in a tub of water, and keep squeezing the lever until all the air’s purged from the system. The risk there, according to some Magura guru-types, is corrosion.

Learn something new everyday

I have never seen hydraulic brakes that didn’t use some type of brake fluid, but if John says they use mineral oil, he’s usually right.

The point was to be sure, from a manual or site, what type to use. Most cars and motorcycles will have it stated somewhere on the fill cap.

It’s good that you seem to have fixed it. Calipers do benefit from disassembly and cleaning every few years. I have also seen them messed up by spraying wd40 on the pistons. If they start to drag, usually it’s the piston sticking a tiny bit.

maguras use mineral oil, as do some other bike brakes.

I would worry about disassembling the calipers, as the manual says never to mess with them.


Yes magura specifically mention that you should not use DOT fluid, as it will affect the seals.

The other risk with water is that it will freeze, causing your system to rupture. Magura reccomend water if you’ve got no other option, but suggest that you flush and refill the system with magura blood asap.

Difference between motorcycle and bike brakes

In motorcycles, brake fluid is replaced because it absorbs water from the atmosphere. So it is changed yearly.

This water in brake fluid is considered super bad on a motorcycle. Brake friction heat moves from the pads into the calipers. If there is water there, it will boil, locking the brakes.

Unis, with our super low rpm, and low load, never get that hot I guess.

I would think that some down hill bikes could boil water. I haven’t worked on any. I have never heard of using water as brake fluid. For that matter, brake fluid is cheaper then water in many places.

They always say never to rebuild their stuff, buy new stuff. I would expect any caliper used outside to get gunky and sticky. Trucks seem to last longer then cars, and motorcycles need work 3x as often as a car. I would expect tiny bike calipers to need the most work of all. It’s just a piston in a can, sealed with a rubber ring. This pushes against the brake pad. They are all the same, more or less. Some fancy set ups will have many pistons etc. Hydraulics is pretty simple.

Car and truck calipers are often a big messy job to clean and reassemble. The cost of a factory rebuilt item is low.

For old motorcycles, and I will guess bikes, parts for these little things are costly, compared to car parts. Because the parts are small, the cleaning and reassembly job is fast and worth doing. Or just buy new parts. In any event, don’t expect calipers to work so great after a few seasons in the great outdoors. Crud builds up around the pistons and the brakes start to drag.

interesting that you say this, coming from the world of car circuit driving the fear with boiled brakes is that the brakes will be ineffective as there is gas in the line which is highly compressable. The effects are most common when returning to the pits, as heat soak sets in. I’ve seen guys run in to other cars in the pits at 5mph because their brakes weren’t effective enough to stop them.

I’ve had mtbing mates who’ve boiled their brakes with DOT fluid on serious downhills, the advice to use water always comes from trials bikers who of course put virtually no heat in to their brakes.

Locking the brakes

I have seen many cases of locked brakes on disk trucks, cars and motorcycles.

The surface between the brake piston and the caliper cylinder collects dirt, corrosion, dried brake fluid, etc. This makes it want to stick. Prolonged braking, say an overloaded car going down a mountain, will boil water in the brake fluid and push the dirty piston out and it gets stuck.

So it’s about dirty brake pistons getting stuck. Anyway, I would buy new calipers for a car or truck. I could fix them, but it ain’t worth the mess.

I just want to encourage those with older bike disk brakes to consider that proper disassembly , cleaning and reassembly, is likely to be not much of a mess, and should restore new performance.

I have never actually worked on any bike disk brakes. :slight_smile: