Last week we dealt with fitting disc brakes in an ideal world where everything works out just fine straight from the box. But what happens if you need to trim cable? And what on earth is bleeding when it refers to brake bleeding and not the holes in your knees and elbows?
So although we there won’t be any “explicit adult content” on here we suggest you reach for the mopping up rag as we take a firm hold on your one-eyed braking snake.
Before we go any further, we’re sorry for the shortage of pictures on this, but trying to balance a camera, brake fluid, bike and various hoses and lever pulls just didn’t work in the trickier moments.Animal, mineral or vegetable?
As always, the first thing to do is to make sure that you have the right equipment for the job. Start by checking what hydraulic fluid your brakes use, as the wrong fluid can ruin the seals in your system very quickly. Shimano and Magura brakes use a mineral oil which is friendlier to your paintwork and the environment but is harder to find away from your bike shop or your local Citroen garage. Hope, Hayes and the others use the synthetic braking fluids that most garages stock, but are less paintwork and flopsy bunny friendly. Makes sure you get the right ‘DOT’ weight (normally 4.1 or 5.1) as performance varies according to their ability to tolerate high temperatures.Short back and sides
The last thing you want to do when riding is to tear your delicate hose off on a big branch, so you need to tuck it in neatly. Most brakes come pre-assembled and ready to install but as they’re designed to fit even the extra long travel forks of downhill bikes you’ll probably find the hose is too long for most XC bikes.
Again preparation is key – so check the instructions to see if you need a new connector to plug into your trimmed end. If you do don’t try and bodge it with the old one – from experience it makes a ruddy awful mess.
Once you’ve got all the fluid and fixtures you need, measure the correct length – making sure you leave enough slack for the bars to twist all the way round in both directions without crimping the hose.
Slicing style depends on the brake hose. Plastic hoses can be trimmed with a very sharp knife, but hold them steady on a chopping board to stop them flying off and creating a crooked end. The less fluid you lose the less trouble you’ll have bleeding so try and keep the cut end upper most at all times. For this reason it works best to trim the lever end not the brake end.
Avoid taking your knife to braided metal hoses though as they normally use specific weaves at the end of the hose and are best pre-bought in the right length.
Once you’ve snipped, take your time to make as good a connection as possible back into the lever end, as the last thing you want is to have it leak or pull out halfway through a ride. Most systems use the same hose pushed over connector and then clamped on tight system as your garden hose pipe and the technique is just the same. Ram the hose onto the fir tree as far as possible before tightening down the locking collar.Little Bleeders!
You should now have a sealed hydraulic system that’s the right length and securely plugged in at both ends. The trouble is all that chopping and changing will almost certainly have let some air into the system. As the air (unlike the hydraulic fluid) can compress, this makes the brakes feel really spongey and powerless. Any air bubbles left in will also expand with braking heat and lock the system up very quickly, so like central heating radiators, your brakes need bleeding. There are various way of doing this and several manufactures even produce handy kits to help, but this is the way that always works OK for us and has the added advantage that you can do it with the minimum of tools in an emergency.
Over time even untouched braking systems will also leak slightly, which can let in air, water or even grit into the system, so it’s worth treating yourself to fresh fluid every year or so.
Whether you have an open (rather confusingly this means a system that uses a sealed self compensating reservoir such as Hope Mini, Enduro, M4, Formula, Hayes or Shimano) or closed (systems where you control the reservoir capacity and therefore lever reach – Hope C2, XC4, Giant MPH) the technique is the same.
First take out the wheels and lock the frame securely into a workstand or the jaws of a passing dog of helpful disposition, making sure the lever is as high above the caliper as possible to let bubbles rise up easily. Then you need to find the reservoir and move the lever so that the top cap is flat and level to stop slop. As most brake fluid is evil, nasty stuff we recommend you wrap as much of the frame, bars and brake levers themselves with kitchen roll or similar to mop up any spillages.
Now carefully take the lid off the reservoir, and you’ll find a rubber seal underneath which you also need to take out, put both of them safe and clean in a container to stop the brake fluid damaging any valuable antique furniture.
To get any air out of the hose, start at the caliper end and start flicking the hose with your fingers to dislodge the bubbles. Work your way up to the lever giving the hose a good tap every inch or so and you should see bubbles rise into the reservoir. Work back and forward along the length several times to get as many out as possible. Sometimes this “burping” is enough to clear the system of trapped wind (in which case top up the reservoir and shut the lid) but there may be air trapped that only comes out when you flush it through properly.Feeling flushed
To flush the system though you need to open it at the caliper end. But before you do anything, take the brake pads out to stop the risk of them being covered in fluid and ruined. Then take a piece of pipe that fits firmly and securely onto the outlet valve of the caliper (normally next to the inlet valve with the brake hose on) and stick the free end into a jar or something else that can hold the waste fluid. If you’re worried about knocking it over on your best shag pile then gaffer tape it to the chainstays to hold it in place.
At this point another pair of hands is massively useful but only if they’re attached to a brain with at least the basic ability to operate a spanner on demand and remain undistracted from this task for several minutes.
Now you just open the caliper valve enough to let fluid flow, and pull the lever gently to pump fresh fluid into the jar, taking trapped bubbles and other choss with it. Hold the lever fully in and then get your glamorous assistant to close the caliper valve so you suck fresh fluid from the reservoir rather than the sump jar. If the reservoir needs topping up then do it, and keep pumping and sumping until no more bubbles or filth appear.
Pulsing the lever in and out slightly, rather than just pulling pints can help shift stubborn bubbles, but be careful not to pull too hard and squirt fluid into your eye from the reservoir, or undo the caliper valve too much and leak fluid all over the caliper, frame and floor.Done job
Once you’re sure you’ve got all the air out then top up the reservoir as full as possible and gently pop the head seal and reservoir cap back on. If you’ve got a closed system with a pad adjuster dial on the top cap then make sure that it’s backed off as far as possible before you put it back on.
Now carefully clean any spilt fluid off the bike or yourself, and put the disc pads and wheel back in. Check all the valve bolts are securely closed and then have a test pull on the lever. The lever feel should be firm and direct with little sponginess after the bite point as you apply pressure. If the lever still squidges back towards the bar then repeat the bleed. If the pads are too close together to let the disc rotor rotate then check the pad adjusters are backed off if it’s a closed system and then carefully bleed a little bit of fluid from the caliper and re-close the valve before releasing the lever.
You should now be the proud owner of a set of perfectly operating disc brakes.Next Week
Now you’ve got them set up how do you keep them sweet? Regular checks, pad replacement and rotor cleaning will be keeping us busy next Monday.