Author Topic: Stuck master cylinder piston - How to get this out - 71 CB750  (Read 16634 times)

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Offline ksmith0034

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Stuck master cylinder piston - How to get this out - 71 CB750
« on: November 14, 2010, 11:42:29 AM »
It appears that the piston is stuck in the "depressed" position on my '71 CB750 master cylinder.  Any ideas how to get this out without destroying the rubber boot?
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Offline TwoTired

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Re: Stuck master cylinder piston - How to get this out - 71 CB750
« Reply #1 on: November 14, 2010, 01:02:58 PM »
The rubber boot just pops off.  It has a wire ring holding it on with friction and groove to capture the rubber on the piston stem.
You have to get the boot off, to access the snap ring which blocks piston extraction.

You can use hydraulic pressure to push the piston back out until it clears the feed hole in the reservoir.
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

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Offline ksmith0034

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Re: Stuck master cylinder piston - How to get this out - 71 CB750
« Reply #2 on: November 14, 2010, 01:20:41 PM »
You can use hydraulic pressure to push the piston back out until it clears the feed hole in the reservoir.

how do I do that?
It took 10 years but she's finally done!
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Stocker 2 Rocker CAFE conversion

My YouTube channel HERE:
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Offline Rob M

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Re: Stuck master cylinder piston - How to get this out - 71 CB750
« Reply #3 on: November 14, 2010, 08:20:21 PM »
Mine, I got the little ring holding the rubber boot off with a little mechanics pick, then with the boot off you'll see the circlip that was a real bugger for me to get out. Longest thinnest circlip pliers you have.  Once they are both out the shaft will pull right out, but the good news for you is with the shaft depressed like that it will make this whole thing a lot easier, I had to keep trying to hold mine in while trying to get the circlip. If you don't already have a rebuild kit for this, I'd get one quick, it won't be something you will want to turn around and do again in a few months. Huge PIMA.  Good Luck.

Offline TwoTired

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Re: Stuck master cylinder piston - How to get this out - 71 CB750
« Reply #4 on: November 14, 2010, 09:47:53 PM »
You can use hydraulic pressure to push the piston back out until it clears the feed hole in the reservoir.

how do I do that?
Use another master cylinder and hook up a brake line between the two.  I use a spare I have laying around.  But, you could "borrow" one from another bike.

Cheers,
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline ksmith0034

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Re: Stuck master cylinder piston - How to get this out - 71 CB750
« Reply #5 on: November 15, 2010, 10:47:10 AM »
I gently pulled the rubber boot off, then was able to carefully persuade the piston back out with a screwdriver.  After a minute of working the piston bask and forth it actually moves very smoothly now.  So I filled up the reservoir, connected the lines and started the process - only to find brake fluid running out the piston end of the master cylinder.  Doh!
Looks like I either need to buy as rebuild kit of just buy one of these from David silver Spares:


http://www.davidsilverspares.co.uk/CB750K1-FOUR-1971/part_123911/
It took 10 years but she's finally done!
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Offline greenjeans

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Re: Stuck master cylinder piston - How to get this out - 71 CB750
« Reply #6 on: November 15, 2010, 02:54:48 PM »
Get a rebuild kit.  Cheap.
Yep, I'm the kid that figured out how to put things back together...eventually.

Offline ksmith0034

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Re: Stuck master cylinder piston - How to get this out - 71 CB750
« Reply #7 on: November 15, 2010, 03:19:01 PM »
Get a rebuild kit.  Cheap.

Who do you recommend?
It took 10 years but she's finally done!
             ↓   ↓   ↓   ↓   ↓
Stocker 2 Rocker CAFE conversion

My YouTube channel HERE:
https://www.youtube.com/MotoCafeRacer

Offline CrankyOldGuy

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Re: Stuck master cylinder piston - How to get this out - 71 CB750
« Reply #8 on: November 16, 2010, 04:17:54 PM »
Just butting in to this thread.  ::)

Does anyone know if the DSS replacement master cylinder is made of metal or plastic?  The one on my 750K1 suffered a bad case of road rash in an earlier life and the anodizing has gradually faded away.

Thanks

Harry O.
750 K1 Original Owner

Offline TwoTired

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Re: Stuck master cylinder piston - How to get this out - 71 CB750
« Reply #9 on: November 16, 2010, 05:42:35 PM »
Get a rebuild kit.  Cheap.

Who do you recommend?
I've always gotten mine through Honda.
Lloyd... (SOHC4 #11 Original Mail List)
72 500, 74 550, 75 550K, 75 550F, 76 550F, 77 550F X2, 78 550K, 77 750F X2, 78 750F, 79CX500, 85 700SC, GL1100

Those that learn from history are doomed to repeat it by those that don't learn from history.

Offline BeSeeingYou

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Re: Stuck master cylinder piston - How to get this out - 71 CB750
« Reply #10 on: November 16, 2010, 11:46:55 PM »
Get a rebuild kit.  Cheap.

Who do you recommend?
I've always gotten mine through Honda.

yeah, spend the extra $10 and get the OEM ones and skip the cheaper Ebay generic stuff.

Offline dave500

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Re: Stuck master cylinder piston - How to get this out - 71 CB750
« Reply #11 on: November 16, 2010, 11:51:43 PM »
silvers has genuine

Offline CrankyOldGuy

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Re: Stuck master cylinder piston - How to get this out - 71 CB750
« Reply #12 on: November 17, 2010, 03:41:30 PM »
Dave,

Thanks

Harry O.   :)
750 K1 Original Owner

Offline ksmith0034

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Re: Stuck master cylinder piston - How to get this out - 71 CB750
« Reply #13 on: November 17, 2010, 03:49:44 PM »
They actually sell original and repop
It took 10 years but she's finally done!
             ↓   ↓   ↓   ↓   ↓
Stocker 2 Rocker CAFE conversion

My YouTube channel HERE:
https://www.youtube.com/MotoCafeRacer