Master Cylinder Rebuild

[inanecathode]

My first try at a how to! Bear with me!

Remove the mirror from the master cylinder body

Remove the brake lever pivot bolt and lock nut

Carefully remove the front brake switch

Place a rag beneath the brake line fitting

Crack, but do not remove the brake line

Remove the master cylinder mounting bracket

With your rag in place, remove the brake line and banjo bolt (catch those crush washers!)

Hold the brake line up, and simoultaneously tip the master cylinder backwards to keep the fluid in it (this is to keep the brake line
reasonably air free, and to retain some of the brake fluid in the cylinder

Take the master cylinder over to your work bench, and somehow keep the brake line verticle (dont let it drain out)

Tip the master cylinder over a pan to catch your brake fluid

Remove the cover screws (might have to stand on the screwdriver a bit to get them out, they're really tough.

Drain the fluid the best you can out and into a pan

Reach in the cavity where your master cylinder piston sits (where the brake lever pushes, you did remove the brake lever didnt you?) and pull out the dust boot CAREFULLY. You might have to pry up at the base with a pick. Watch out though, it is quite fragile.

Now. This is the hard part. Take your long skinny internal snap ring pliers (ones that squish when you close the grips, you do really need skinny ones to get down in there) and remove the snap ring that holds the master cylinder in. It IS quite hard to do, but keep at it, it does come out.

When you do get it out, it'll probably come flying out and roll under the hardest to reach item in your work area, contain that beast with a rag.

Lay out your parts in the order and direction they came out of your cylinder, they should be as follows from left to right:
Master cylinder
Piston return spring (cone shaped affair, wide end goes to the left)
Master cylinder piston (looks screwy through the middle)
     Primary cup (goes on the little shouldered end of the piston)
     Secondary cup (goes out towards the middle right end of the piston)
     NOTE WHICH WAY THESE FACE! The open end should face left, into the cylinder!
Snap ring retainer
Dust cap

Take your two cups out of your master cylinder rebuild parts bag, pop them in a cap full of fresh brake fluid for a few minutes, while you're at it, put your new piston in there too

Take a break, go get a wobbly pop, cause you're going to need it

The primary cup (goes on the little shouldered end of the piston) i put on first, it should just pop right on. Make sure the open end is facing out towards the brake line/system/cylinder

Now on to the secondary, yes, it is supposed to go over the entire piston, then into the groove you see that holds the secondary on your old piston. Just carefully (and MAYBE with a VERY dull screwdriver) gently pry/nudge/push the cup onto the piston into the groove that holds it. It DOES go on, and it DOES go on without tearing if you're gentle enough

Your cups ARE facing the right way right?

Now put your spring onto your piston, the skinny end goes onto the front end of your piston (the end with the cup closest to the end) set it aside

Look into your master cylinder bore and surround. It should be very very clean, and very very smooth. If it isnt, make it so. I wouldnt touch the cylinder bore much though, its a very finely machined and honed surface. If it's wicked pitted, you might be looking at a new master cylinder body.

Once your body is all nice and clean, and so is your master cylinder body (really, should have clean hands when doing this), lubricate your piston and your cylinder thouroughly (can't do it dry kids :D)

With your spring attached, work in the primary cup into the cylinder bore using small circular motions whilst gently pressing (linear shoving might roll over your cup, bad times)

Once the primary cup gets in hook your snap ring up to your snap ring pliers, place it around the piston

Now push down to the secondary, and work this one in too in small circular movements (this one is a bit tougher to get in, don't force it, but it is a bit tough to push it in)

Take your pinky finger, and hold the piston down in the cylinder

Squeeze your snap ring inwards, and get it to seat down in the groove (this is pretty hard, your piston will probably come out alot, but it does go in)

Once the snap ring is seated, pop your dust boot back on. Big end seats in the master cylinder body, small end seats on the piston end

Over your pan, pour a bit of brake fluid into where the brake line hooks up and plug it with your thumb (you... are wearing gloves arent you? brake fluid causes testicular cancer in lab rats, just so you know)

Pour in a bit of fluid into the resivour with the master cylinder upwards

Depress the master cylinder piston, there should be a bit of pressure on your thumb

Release your thumb whilst depressing the piston, fluid should come out

With the piston depressed, plug the hole and let the piston return, the fluid level should drop slightly

If this happens, congrats, you've got a rebuilt cylinder!

The installation is the reverse of removal, except dont tighten down the brake line, just get it on the cylinder within about 2 turns of seating

If you left your line reasonably upright, you can skip this next step, if not, read this next part

     Put a syringe full of brake fluid on the caliper bleed screw, apply pressure. With the brake line at the master cylinder cracked, crack      the bleed screw and push down on the syringe until no more air escapes the line, close the bleed screw

Crack the brake line at the master cylinder, slowly depress the brake handle (you... did reassemble the brake handle didnt you?). once the handle bottoms, close the banjo bolt, and return the handle

Repeat the above step until the brake feels reletively hard. If it still isnt, bleed the caliper and line and retry. This does work, so just keep at it

Once you have the brake working reletively well, traditionally bleed the system extensively, and you're done :D



Questions, comments, additions, subtractions?

[kwells]

uhhmm....
what's a perch

[inanecathode]

uhhmm....
what's a perch

fixed :D

[kiawrench]

 a small trick for getting cups on and not tearing them --- i put celophane film along the piston, a long piece, maybe six inches long then feed cup over that, and slowly use it to guide the cup into place   it gives a wider surface to wiggle the cup on and is safer than usng a pick
once cup is where you want it ,a small nick with a razor blade will split the film and allow it to be removed  quickly
you can get celophane film from any photo shop, it is used as a leader for those one hour photo developer machines , and they have plenty in the trash can - just wipe it down with brake fluid and a clean rag - then lube it up and go with it .