Rebuilding your brake master cylinder - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 13 (permalink) Old 10-20-2011, 10:13 PM Thread Starter
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Rebuilding your brake master cylinder

Since I had to go about rebuilding my brake MC, and I love knowing absolutely everything about something before diving in to fix it, I took some pictures and documented everything.

What you need:
Flat head screwdriver
Phillips head screwdriver
10mm socket
12mm socket
5(mm?) allen wrench
longish circlip pliers
Swiss army knife pick

what is super helpful:
socket extension
syringe from vet store or flavor injector
garbage bag to cover bike from brake fluid


At first I thought I could do this with the MC on the bike. If you have before, you probably can, but it is soooo much easier off the bike.

Rip a hole in a garbage bag and slide the mirror and right controls through it.

Take off brake lever with 10mm socket on the bottom and flat head screwdriver on top.

Open the reservoir and use syringe to draw out all the fluid. Replace cap.

Use 12mm socket to remove the banjo bolt going into the MC. Careful! There is a very thin washer that may stick to the MC. Hold on to it.

Use the allen wrench to remove the mirror/MC assembly.
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Use the pick to get under the rubber dust cover near where the brake lever was. It may take a little work (don't rip it) since it can be weathered.

Use circlip pliers to nab that circlip.

Slide out piston and spring.

Look at the layout of everything in front of you including the order. No surprises that way going back in.
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Looks simple enough. Rubber with dimple mates to a hole on the spring. Put that in the MC. Use the swiss pick to get the rubber ring on the piston. You have to stretch it a lot, but it can take it. Just don't use the point and make sure it is facing the same direction as the one you took off. Hold in piston and put on circlip. Dust boot back on. Voila! You dun it! Toss it back on and bleed the brakes.
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post #2 of 13 (permalink) Old 10-21-2011, 11:14 AM
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This should be a sticky. Good work.

I'm keepin' all the left over parts. I'm gonna use 'em to build another bike!
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"Black Beauty"
1989 VN750 acquired December, 2008, 6,711 miles
Currently 23,298 miles

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post #3 of 13 (permalink) Old 10-22-2011, 01:13 AM
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I recently rebuilt all 3 master cylinders, all 3 calipers, and the clutch slave cylinder on my '85 Goldwing. almost $300 worth of parts, including the 4 stainless steel SpeedBleeders. But I got brakes.

I am a motorcyclist, NOT a biker.


1997 Vulcan 750, purchased about a week ago
2006 Sportster 1200 Low
2013 Royal Enfield Bullet 500, converted to carb
2001 Yamaha XT225, heavily modified
2004 Honda Rebel 250
1979 Vespa P200E
2002 Vulcan 750 parts bike
1994 Yamaha XT225 parts bike
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post #4 of 13 (permalink) Old 03-02-2013, 06:38 PM
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Just rebuilt my MC. As I took off the handle, the piston shot out with the spring. So no retainer clip, dust cover or dimple rubber washer on spring. Thought that was why wasn't working, but with it all back together, still not working. Rechecked assembly, all looks good. When I squeeze the handle, some fluid comes out, but not much.
Any suggestions?
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post #5 of 13 (permalink) Old 06-05-2014, 01:19 PM
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I'm getting mine rebuilt right now, for the 2nd time. Brakes seized up on me at the end of last year and am not confident enough to fool with breaks...lol.

1985 750 Vulcan-Red
36,875 miles
New timing chain
New choke cable
Clean front rim
new throttle pull back cable
new spark plugs
New Bridgestone on the back
New Fluids all around
SeaFoam in the tank
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post #6 of 13 (permalink) Old 07-17-2014, 09:17 PM
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Pulling mine apart now, anything I should be looking for to replace when I have it apart?
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post #7 of 13 (permalink) Old 07-17-2014, 11:46 PM
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the kit will come with new piston, spring,seal and should have the circlip. you will need to bleed the master cylinder before doing the calipers.

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2003 kaw vn 750 pushing 40,000 miles.
tp stator
dampers
coil relay mod
other stuff not listed
mustang seat
motorcycle repair at a stealership 1 million bucks, repairs with the help of the Vulcan750 forum priceless.
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post #8 of 13 (permalink) Old 07-17-2014, 11:50 PM
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"if you have the round mc"

make sure fluid is full, diconnect the line from the mc, put you're thumb over the hole, squeeze the brake lever several times slowly. you will see air bubbles come in to the mc and will start to feel resistance. do this until you see no air bubbles. then as you squeeze the lever move which ever finger you have over the hole where the line connects just enough for air and fluid to push out still keeping slight pressure over the hole. do this until only fluid comes out. connect the line back up tighten 1/4 turn. slowly push the brake lever in till you meet resistance. it probably wont be alot yet. then as you push the lever in loosen the bolt on the brake line 1/4 of a turn. when the lever is pressed all the way hold then tighten the bolt. do this until only fluid comes out. now you can start bleeding the calipers. if you have a clear tube to put on the bleeder screw it will help see the air bubbles coming out. i just put the tube in a old bottle sitting on a block so it was slightly higher than the caliper. hope this helps this was the info from the clymer manual. once you think you have all the air out then tie the lever back over night if you have to work the next day dont touch it just leave it tied back. it still may take a day to get full pressure. hope this helps you. it is a pain to do but once you get it done youre brakes will be happy. i would rather pull the motor than bleed brakes on these bikes. after you do it once it isnt so bad.

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2003 kaw vn 750 pushing 40,000 miles.
tp stator
dampers
coil relay mod
other stuff not listed
mustang seat
motorcycle repair at a stealership 1 million bucks, repairs with the help of the Vulcan750 forum priceless.
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post #9 of 13 (permalink) Old 09-27-2014, 08:18 PM
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Cool Avoiding Front Brake lockup

Everyone should get familiar with the Master Cylinder operation to avoid having the front brake lock up on them. I recently experienced this happening due to the return port being plugged in the MC. This was caused from the bike sitting for a long period of time and the corrosion in the MC solidified in the return port blocking the release of pressure to the brake cylinders. What happens here is the pressure will build with an increase of temperature due to expansion of the brake fluid, nothing mysterious here just laws of physics. Some have experience this after riding in the morning and having the brake locked up when they try to ride in the afternoon after the temperature has risen. This can also happen, and has, while riding when starting at a cooler temperature and riding into a warmer temperature area. The fix is simply cleaning the MC and clearing the return port. The return port size is about 0.015" and is the furthest from the brake lever. This should be accompanied by a complete brake fluid change. This can be prevented by maintaining clean fluid. Each time the brake lever is pulled there is a little fluid pushed through the return port which will help not letting it solidify. Letting dirty fluid sit for a long period is a problem for this port and the rider if he is not aware. Ride Safe!
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post #10 of 13 (permalink) Old 05-02-2016, 09:43 AM
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Video showing proper operation of a master cyl when cleaned. If your brakes lock up this is likely why. Soot gets down in the valve and stops it from producing air.



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