my rear brake mstrcyldr piston is stuck kz1000 '78

Hi, below are photos of the master cylinder for the rear brake on my 1978 kz1000 A2 (standard) model.

The piston in the master cylinder shaft is stuck!
I think normal operation has a spring push the piston back out.

Pushing the brake pedal down moves the piston upward towards the round brake fluid reservoir -- and that's where she stuck! In the 'fully depressed brake pedal' position.

I've already munged the front master cylinder rebuild, I want to make this work somehow using cleverness and avoiding disassembly of the piston -- any ideas?

I'll listen to ALL ideas though! NEED REAR BRAKES NOW!

I guess what I'm looking for is how do I apply pressure to the top side of the piston so it slides back into its 'fully resting' position? My thinking is, I can then rock the piston back and forth, and I've already fill the reservoir and the piston chamber with fresh brake fluid after cleaning them out.

Thanks!

probably a bunch of gunk/rust in bore,id say bite the bullet and pull it apart.you might get it freed up only to have it seize up again at a stoplight miles from home if ya dont go through it.

PLUMMEN wrote:

probably a bunch of gunk/rust in bore,id say bite the bullet and pull it apart.you might get it freed up only to have it seize up again at a stoplight miles from home if ya dont go through it.

Yeah. What he said. :laugh:

...and once you clean it out all nice nice, it'll work so much better.

Looks like you cleaned parts on the Master cylinder with water. Go with Plummen's advice and take it apart. Be sure you get the moisture out of it.

unomike wrote:

Looks like you cleaned parts on the Master cylinder with water...Be sure you get the moisture out of it.

Nope the bike set 10-12 year and that wee bit a rust is not nearly too bad as the rest of the beast! Have not cleaned this one, only whined to m'self about this-n-that in the disassembling of it, woe is me. Damn things, before starting the disassembly I've made plans to re-locate my sledge hammer and re-injure my right big toe, that'll teach my dumass fer gettin a 'fixer' !

I have no skills with these dam brake rebuilds apparently, the pistons get so hard stuck in there.

I have a parts diagram -- HOW DO I COAX THE PISTON OUT OF ITS BORE? I don't see a way to tap it out from the topside, the bore is closed off there, it only has an opening on the piston end shown in my pix above.

HOW DOES ONE GET THE STUCK PISTON OUT THEN?

Could remove retainer clip and apply compressed air (into exit hole where brake line attached) to blow the piston out.

WARNING : Piston may shoot out like a bullet, so aim it accordingly in a safe direction, such as perhaps into a folded shop rag. Do NOT hold the shop rag in your hand -- Duh -- think Quinton Tarrantino in From Dusk Till Dawn). :pinch:

Would also invest in a genuine Kawasaki factory service manual covering the particular model in question.

Good Luck!

I posted a fix two weeks ago using a customized bolt sized to fit into threaded opening where banjo bolt is. It has a grease fitting threaded into the top of it so a grease gun can be used which is more controlled than air. The grease can easily be washed out later with solvent. This fix is suitable for most bikes from '73 to '80 as fittings I believe are all the same, cheap too!

here is the previous thread.

kzrider.com/index.php?option=com_kunena&...ew&catid=5&id=336203

Good idea Kawboy! Ill remember that! A ring of corrosion and oxides always develop over time locking the MC piston inside the bore. I use a mild acid solution(vinegar)and follow up with a wire toothbrush and penetrant, you can then grab the piston and pull it out with the tool of your choice.Needle nose vice-grips work well and any bite marks on the plunger stem can be buffed off.

zoro wrote:

Good idea Kawboy! Ill remember that! A ring of corrosion and oxides always develop over time locking the MC piston inside the bore. I use a mild acid solution(vinegar)and follow up with a wire toothbrush and penetrant, you can then grab the piston and pull it out with the tool of your choice.Needle nose vice-grips work well and any bite marks on the plunger stem can be buffed off.

Various things in life scare me: waking up and finding myself still married to my crazy ex-wife; or waking up and discovering my Navy discharge didn't go through back then and I've been AWOL 20 years and the MPs just found me; or MC pistons flying out of master cylinder shafts like bullets and exceeding the design limits of 'rags stuffed into a piston-catching box.'

However, I like the rapid fulfillment of a dream which is instant expurgation of the stuck piston as it yields to my 120psi compressor but -- I do not know how to connect my air hose to the threaded banjo bolt hole. If I knew the answer to that I'd ignore the bulletization concern and go for the air gun's 'will-not-be-denied' influence.

And that grease gun fitting sounds effective.

But hey I love this 'vinegar and penetrant' -- would that be 'penetrating oil'?

And where do you grab on to the piston with needlenose? See my pic above -- doesn't seem to be anywheres to grab it that way??? And how long do you allow the vinegar then the 'penetrant' to set? 1 day? More? Less?

The piston I was referring to is the 5/8" master cylinder piston. My caliper pistons were no problem. I grabbed the MC piston at the end of the plunger with the dust seal removed.The vinegar is added when the dust seal is removed and allowed to eat at the oxide ring for 1 or 2hrs. The penetrating oil simply aids in the separation process of pulling the piston free from the bore.I expect any mild acid solution will attack the corrosion more readily than the surrounding aluminium as long as you dont leave it on the part "too" long or use an acid thats too strong. I then of course followed up with a thorough cleaning and servicing.My apoligies!I failed to see your pics before I commented.I was refering to the front MC:blush: The clip retainer must be removed followed by the stop washer,now the acid can be used if the piston proves stubborn.In your case, I would soak the whole unit over night in gasoline and then begin working the piston free with the pushrod while the clip and the stop are removed. Remove all the fittings and cap before the soak. What you have is a piston assembly full of muddy crap keeping the return spring depressed.Gasoline will dissolve and loosen the crud while repeated taping and pushing should free the piston.Do keep us updated on your progress. Merry Christmas!

zoro wrote:

The piston I was referring to is the 5/8" master cylinder piston. My caliper pistons were no problem. I grabbed the MC piston at the end of the plunger with the dust seal removed.The vinegar is added when the dust seal is removed and allowed to eat at the oxide ring for 1 or 2hrs. The penetrating oil simply aids in the separation process of pulling the piston free from the bore.I expect any mild acid solution will attack the corrosion more readily than the surrounding aluminium as long as you dont leave it on the part "too" long or use an acid thats too strong. I then of course followed up with a thorough cleaning and servicing.

Got it, I have some malt vinegar put on the fish, will use that for 2 hours tomorrow. I think I'll have a time finding something on the piston to pull on with the needlenose because it's descended into the MC bore (see pic above) and theres no outcrop on the end of the piston to grab hold of, if the MC piston wasn't stuck so deep inside the bore, if it was sticking out the end of the bore a bit so I could grab the edge of it....so I guess I'm still missing something about what you grab onto when the piston's stuck deep inside the bore, guess I'll have to hopefully have a breakthrough tomorrow....THANKS ABOUT THAT THERE VINEGAR! I already have my wired toothbrush, looking forward to making progress.

I only hope the rebuild kits for this'n don't cost the $45 that the front rebuild kit cost.

newOld_kz1000 wrote:

... I do not know how to connect my air hose to the threaded banjo bolt hole. If I knew the answer to that I'd ignore the bulletization concern and go for the air gun's 'will-not-be-denied' influence....

Use a blower that has a rubber cone tip.

Good Luck!

[Click on image to see MP's arrest crazy ex-wife.]