Gen 1 leaky rear master cylinder

[Lez72]

I am currently in the process of giving my Gen 1 a deep clean and a bit of a service etc. I fitted a rear 'Venhill' brake line to it today, as I did the fronts a few days ago, and then set about the arduous task of bleeding her up. I managed to get nice fresh, bubble free fluid from both bleed nipples on the rear calliper but I still wasn't happy with the lever pressure. There was too much travel in the lever before the real pressure started.

After about an hour of pumping the pedal and bleeding at the calliper end I gave up. I inspected for leaks and saw a slight misting of fluid around the base of the rubber boot that sits at the bottom of the rear master cylinder (the bit where you adjust the lever height). Its not dripping out and you really have to push your finger into the underside of the boot to get a minute amount of it on your digit, however, i'm guessing its new master cylinder repair kit time ?

I just wanted to know before shelling out the £56 that AJ Suttons have the repair kit listed for  :'(

I can't really say what the lever pressure was like before I fitted the new brake line and bled it, as I haven't ridden the bike for a while due to using my boxeye for most of last year. It was fine when I last used it obviously. The Gen 1 has only done 10,000 miles so I am a bit surprised that a 'Brembo' part has failed within this mileage.

Has anyone else had similar issues at this mileage and am I right in thinking a repair kit is required ?

Thanks in advance.

[Falcon 269]

Before you do anything with the master cylinder, have you tried cracking the Venhill fitting at the MC end to bleed any air out of the system at that point?  Obviously you need to prevent any fluid coming from the joint from squirting onto the paint or swingarm finish.

Rear MC seal failure isn't something we hear of often.  If you're still not happy with brake lever feel after trying the above, you might find it cheaper to buy a used one complete off ebay.

[Lez72]

Thanks for the advice Mike.

I haven't tried bleeding from the master cylinder end but I will give it a go. I did think about getting a 2nd hand unit and had a quick look on Ebay to get an idea of prices. The general rule seems to be between £20 - £30. I just didn't want to buy one from a 30,000 mile bike when mine seems duff after 10,000 and that's what led me on the train of thought of repairing what I already have rather than buying another to find that is duff too.

Is a bit of fluid residue under the bottom rubber boot of the Master Cylinder normal ?

Regards.

[Falcon 269]

Is a bit of fluid residue under the bottom rubber boot of the Master Cylinder normal ?

To be honest, I don't know - I've never looked at mine to check.    

I'd clean and dry the area thoroughly and see if any misting occurs again.  It could be that you're tramping really hard on the lever striving for a solid feel and that's maybe forcing a little fluid out. 

Like I said, rear MC failures aren't common so buying one of ebay shouldn't be a gamble.

[The Male Whale]

If the travel was OK before new line was fitted, then it would be a long shot that the MC failed at the same time you fitted a new hose....


"When hearing hoof beats expect to see horses and not zebra's."


To test the MC, take the hose off at the MC end, shim the banjo up with the copper washers to block the holes and screw the banjo back in. If you now have travel it's the MC, if you don't it's air.


Air in the line my guess - follow what Mike says or remove the caliper, sprag the pads and bleed the caliper nice and high above the MC.


Whale

[Falcon 269]

If the travel was OK before new line was fitted, then it would be a long shot that the MC failed at the same time you fitted a new hose....


"When hearing hoof beats expect to see horses and not zebra's."

Absolutely logical.  And I love that quote every time you use it, mate.

[Lez72]

All of the above makes perfect sense and I too thought I would have to have been unlucky for the master cylinder to have failed when it did but I wondered if it had occurred due to me pumping the pedal a kazillion times whilst trying to bleed it up. Prior to reading all of the above I have took all the rear brake components back off today and stripped everything down including the master cylinder and rear calliper to see if there was anything obvious amiss. There was some brake fluid residue in the rubber boot on the master cylinder but all the seals and plunger look ok. I guess its hard to tell if a seal is not seating right or is worn but there didn't look to be anything obvious. Anyways, I have cleaned and blasted everything through and the master cylinder boot is now dry as a bone. I am going to get 2 new bleed nipples tomorrow and then put it all back together and give it another go as I think one of them might be seeping slightly.

Thanks Male Whale regarding the tip on how to check the efficiency of the MC. I like it  8)

I'll post again when the issue is solved. Thanks again guys 

[The Male Whale]

Absolutely logical.  And I love that quote every time you use it, mate.

If I have made you happy Mike then my work here is done!  :lol


Whale

[Lez72]

All sorted now guys and thanks for the advice. After I had cleaned everything through and fitted 2 new bleed nipples today I set about pumping some freshly opened fluid through the system and 20 minutes later its all sorted and good as new. I started off by squeezing the air from the rubber hose that leads from the reservoir to the MC and the rest was a breeze. The rubber boot has remained bone dry too so maybe everything just needed a thorough clean out and re seating.

Best regards.

[Falcon 269]

Good news!