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Hi all has anyone got any ideas what's wrong with my rear brake master cylinder for a 1980 gs 850 , firstly the bike is now a trike with reliant robin axle and drum brakes all new shoes and wheel cylinders. The  reservoir had dirty fluid in so i thought id change it , I used a syringe to get mots fluid out without getting air in the system filled with fresh fluid and then tried to bleed rear brakes in the normal fashion, but no fluid would pump out , so i made a vacuum with a  compressor to draw fluid out but then nothing came out and just squashed the pipe at bleed nipple. i thought something then was blocked so stripped it all down all pipes are clear stripped rear master cylinder , rubbers look ok nothing is blocked no leaks or scores in cylinder seemed all ok , put it all back , first used syringe to back fill rear brakes and pipes with new fluid till it came out of bleed nipples, locked them up then filled inlet and banjo outlet with fluid before tightening them up, connected reservoir and refilled. tried again both normal bleed and vacuum but still the same no brake pressure wont bleed and no rear brakes , gonna try easy bleed tomorrow but have doubts as if it wont vacuum from a bleed nipple as if it was blocked i cant see it pumped from reservoir working either. my conclusion buggered master cylinder or am i missing a trick with this . any ideas anyone  

Edited by Captain Chaos
no buying outside marketplace
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thanks for the reply , ive checked all brake lines all are clear . as its now a trike it only has one rubber brake line that goes from top of master cylinder to a metal tee then on to wheel cylinders all copper pipe lines, ive syringed brake fluid through all the pipes and all are clear. just put master cylinder back on again this morning after filling it with fluid in attempt to prime it. tried again to bleed rear brakes only pushed a few drips of fluid out but did notice as i pushed  released peddle quickly fluid did splash up in reservoir thats all that happens , i cant understand whats going on with this as rubbers look ok M cylinder dont leak its not scored inside or blocked the whole system looks ok there dont appear to be a reason it dont work even back filled it think ill try get another M cylinder from somewhere      

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in your master cylinder there is two holes - one is obvious, the other is a tiny pain in the arse slightly fwd of the big hole- sounds like the small one is blocked ( what you describe is what happens ) you need the thinnest of needles to poke wiggle and generally work its way through this hole - it can be blocked by alloy corrosion crud hence why you sometimes need to work it - I don't stop until I see the tip of the needle coming through into the MC piston chamber

it is important to clear this hole - this is where any brake fluid thermal expansion bleed off occurs - without it your brake can lock on as it heats up

Edited by nlovien
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oh right ive seen one small hole about half way down inside the cylinder would be just before the top rubber with the spring fully extended so there is another much smaller hole am i reading your reply correct to think its further in , lower than the big hole . Just to be certain would this small hole be past by the top rubber  after the bigger hole or vice versa  sorry to be a pain just wanna be absolutely clear  where to look. thank you very much for the info i had know idea there was a second hole , ive never had a problem like this before  car or bike  

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this looks nothing like my master cylinder I dont have a photo of it as yet i cant tell where the holes are in your pic . would they be under where the pipe from the inlet reservoir fits into the M cylinder ? . when you mentioned the small hole i thought you meant actually inside cylinder you can only see that hole after taking piston complete with two rubber seals and the spring out of cylinder. If these holes are under the inlet of the cylinder why would the small hole completely stop fluid getting into the cylinder surly the bigger hole is the feed for fluid and the small hole is just a release of expanded fluid back into the cylinder however i will try to get the feed pipe out and have a look , im a bit dubious of this as mine is like your pic broken on one side and looks fragile but as it is its not leaking so far , more investigating is needed i think and ill see if my local bike shop can get me some new piston  rubbers  thank you for your help im at a loss at mo       

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Is there a little bit of slack in the brake rod? I have seen instances where the brake rod was misadjusted and did not allow the piston to come all the way back past the big hole in the master cylinder and thus not allow more fluid into the system.

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Thanks for your reply strangely i was just thinking of that myself its starting to occur to me that if it wont pump and i cant bleed either normally or with a strong vacuum from a bleed nipple or an easy bleed something is blocked , ive back filled the pipe system with bleed nipples open in turn inc the banjo at top of cylinder , all are free and fluid flows perfectly can only logically be the master cylinder, and with the piston out the cylinder is clear. Put piston back in even without the rod i cant make it work in hand by putting fluid in manually. im also thinking perhaps the rubbers have expanded over the years and are blocking the inlet hole, im positive it can only be for what ever reason it is one of the seals some how covering the hole inside the cylinder. Got to fix this pain in the bum problem as mot is soon, plus i cant find anywhere i can find a new or good second hand one in GB plus the rubber kits dont look the same IE mine has two piston rubbers both fit over piston via centre hole and front one is slightly slimmer than back one, all the new rubber kits i can see have one fit over rubber and a flat sit on rubber (no centre hole)  I know this is an old vintage bike but why are parts so dammed hard to get . if i get this fixed ill put a post on here what i did to fix it in case anyone else get the same trouble . Thanks again for your help it seems to confirm my theory , and thanks for everyone else who have helped with their input 

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13 hours ago, ron said:

this looks nothing like my master cylinder I dont have a photo of it as yet i cant tell where the holes are in your pic . would they be under where the pipe from the inlet reservoir fits into the M cylinder ?

yes - if you remove the "plastic ?" nipple fitment that your reservoir hose fits - what you see in the pic will be similar to whats underneath it 

 

. when you mentioned the small hole i thought you meant actually inside cylinder you can only see that hole after taking piston complete with two rubber seals and the spring out of cylinder.

it does feed through into the cylinder but unless you've got very good eye's you'll not see it - its very small - on the pic its more obvious because there's a dimple on the other side that leads to the hole

 

If these holes are under the inlet of the cylinder why would the small hole completely stop fluid getting into the cylinder surly the bigger hole is the feed for fluid and the small hole is just a release of expanded fluid back into the cylinder however i will try to get the feed pipe out and have a look

see attached picture - can you see how, when you back flush you can not get to the main supply hole because its blocked by the primary cup seal, the bleed path is only open to the second "relief" hole

b2.png.222d481590b31b1c92e95d285195e862.png

 

, im a bit dubious of this as mine is like your pic broken on one side and looks fragile but as it is its not leaking so far , more investigating is needed i think and ill see if my local bike shop can get me some new piston  rubbers  thank you for your help im at a loss at mo       

 

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ill get it off today and put a pic on here here but im pretty dam sure its the original cylinder , made in japan and fitted in original place etc ill get pics of it and the piston and rubbers a bit later today  

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Also ive found the small hole inside the cylinder below the feed hole and i managed to get a bent pin in it and give it a very good poke until the pin went right in so im confident now that its clear of debris too  found back larger rubber has swollen and is very hard to get back in so gonna try sauce some new rubbers tomorrow   

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that looks like all the used ones on Eblag but they are all in the usa and if you look its priced in dollars so i believe the pic is a usa import , its very very  similar apart from the inlet pipe and the fact the fixing bolt holes are further apart i believe the one i have is for the European market but thanks anyway 

 

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the only bit that looks different is the feed mine is a plastic elbow where this is a straight at right angle to cylinder  but the parts or whole cylinder may be the same , im takeing mine to my local bike shop tomorrow to see if they can recognize this part id like to get new rubbers thanks for the links    

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It may be prudent to invest in a new cylinder too....that way it's all new and you could eliminate rubbers AND cylinder wear.

As for exact type you could simply buy one which has similar dimensions and the amd hole spacings and it should do the job.

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I think ill probably have to do as you say im afraid only problem is finding one there seems to be lots in USA  but finding these vintage parts in GB is a real task had to get carb intake boots from the states , i think all the import ones have a wider fixing hole space so i would have to make some kind of extension to the original fixing bracket . the thing is a dam pain in the ar..e 

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Ron - for the want of maybe a bracket to match the mount holes - that rear brake M/C is just a typical example of just about any of them - noting that its a 14mm bore piston

i.e. maybe adapting any half decent 14mm one could be a better route - maybe your keen to keep things as was

a quick peek on the bay and you'll find chinese assemblies c/w reservoir and brake link for less than £15 - or loads of 2nd jap / european bike ones for not much more

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Hi guys looking at the picture of the seals, they look worn to me. They should have more of an angle from front to rear of seal and a sharp edge where the seals hit the bore. The r/h/s seal looks deformed and stretched. I would change seals. Regards Rob.

 

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yes definitely worn , but dont leak fluid so should pump. the inlet holes in the cylinder are clear, my theory is , the fluid should drain into the cylinder in front of the first seal next to spring . Therefore if with the piston in full brake release position back at the stop , holding it ,it should be possible to blow through it, but i cant or suck through it , i may be wrong but if i cant blow through it then when fitted to bike the fluid wont be able to get in it either therefore it wont be able to pump , which is whats happening , now im thinking these rubbers may well be as old as the bike which is 1980 so i reckon they have swollen the first rubber then blocking the feed hole the second rubber is definitely swollen  as i cant fit it back in any more , im going to 3 bike breakers tomorrow to try get a second hand working unit but last resort ill try get a repair kit my unit is old and bits are chipping off it so a better second hand one would be cheaper than a new unit thats very expensive     

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