Priming the Master Cylinder

[snatchpiece]

I've renewed the seals in the rear master cylinder, but I cannot get it to pump fluid through, I seem to recall I had this problem a few years ago on a front master cylinder for another bike, I overcame it by furiously working the lever back and forth till very tiny air bubbles appeared in the reservoir and then all seemed to work ok and I could bleed the system, I've tried this on the Morini, working the brake pedal until my arm nearly dropped off but to no avail. Any suggestions?

[Steve Brown]

The easiest way is to draw the fluid through with a vaccuum. If you fit a peice of tube to the bleed nipple and open it, then apply a light vaccuum to the system the fluid should feed through quite nicely and without air bubbles-as long as you don't let the reservoir run dry. If you get hold of a 'Mityvac'- it's a hand held pump with lots of uses- they come with adaptors etc to make it convenient. You could if you're inventive, rig something up with a bicycle pump to supply the vaccuum.

[snatchpiece]

This is driving me nuts...........I cannot get the master cylinder to pump the fluid through. I've invested in an Eezibleed kit which uses the pressure in a spare tyre (reccomended max 20psi) to pump the fluid through the master cylinder but this has not worked. I've upped the pressure in the spare tyre to 40psi but even this would not pump the fluid through, I dare not use a higher pressure for fear of blowing the non-pressure pipe from the reservoir to the master cylinder. I've slackened the banjo on the feed side of the master cylinder and the Eezibleed is pumping fluid through fine. I've stripped, inspected and rebuilt the master cylinder, everything was OK but I still cannot get it to work.

NLM suggest the same as Steve Brown who kindly replied to this thread, that is to use a vacuum to suck the fluid through the bleed nipple, but if 40psi wont blow it through how will I create a vaccuum great enough to suck it through bearing in mind some vaccuum will be lost through the threads of the slackened bleed nipple. I've looked at Mityvac's on the web but the're quite expensive, it would probably be more economical to throw the towel in and let a m/c repair shop do it.

[Steve Brown]

Hmmm, not surprised you're getting cheesed off by now. Having just read and then re-read your post, the only thing I can think of is the piston seals may be in the wrong way round, perhaps? They will only pump one way, as they are tapered and the wide end needs to face the pressurised side. If not then you will never get it to pressurise the hose and caliper. You say there is fluid at the 'feed' side, do you mean the supply from the reservoir? What is more important is are you getting fluid at the outlet port to the hose? and then at the end of the hose? then the nipples?
You can't be too far away from me (South of Leicester) If you are still stuck, I have a Mityvac of my very own and could arrange something with you, maybe? By the way, mine cost around £30 from M&P a few years ago, cheaper than booking in to a workshop even then. Go for the plastic bodied one, the others are a lot more and meant for professional workshops/heavy users etc.
PM me if you like, and you wouldn't be the first to fit these seals back to front! I was much younger then of course.

[3potjohn]

Have you tried blowing the brake fluid through the bleed nipple to refill the master cylinder with eg a syringe? I re-sealed my master cylinder a couple of years ago-Cannot remember any details though to help.

[snatchpiece]

Thanks for the replies guys. Yes, the seal is fitted correctly, I sent a photo to NLM when I first fitted it and they confirmed it was correct.
I called into a local motorbike repair shop and asked if they would bleed the rear brake for me, "no problem" they said "we do it all the time, by suction", "check there is free play between the pedal and the operating lever to ensure the piston is fully returning" they advised, I did and there was. I took it along but they had as much success as I did, so they stripped the master cylinder and suggested the tapered seal might be fitted in the wrong groove. I took it all home and phoned NLM, I spoke to a mechanic who again told me to check the free play at the pedal, also got it confirmed that the seal was in the right groove.
Left it for a while and did some other stuff, then I had a sit down and a think, I remembered that the Eezi-bleed would pump fluid no further than the inlet to the master cylinder and the pedal had free play, but what if the piston wasn't returning far enough anyway, that would leave the tiny inlet blocked by the tapered seal. I removed the large union from the inlet side of the master cylinder and laid the cylinder on its side with the aperture vacted by the union uppermost, poured a little fluid in and it went nowhere. I gently eased the piston back a little and the fluid dripped slowly through the tiny port into the top of the cylinder, I now had the piston in the right position for bleeding, so I measured the protrusion at the lever end knowing that once re-assembled I would need the piston in this position to be able to bleed the system. Once the operating lever was re-fitted I could not get the piston into the position I needed, I dare not use the Eezi-bleed with the lever detached in case it pumped the piston out of the cylinder, so I re-fitted the lever with a small piece of rod instead of the proper screw, this allowed the lever to drop a bit lower allowing the piston to be where I wanted it. Simply connected the Eezi-bleed, opened up the bleed nipples and hey presto...bubbles galore then pure fluid, tighted the nipples, pushed the piston up the bore of the cylinder so I could re-fit the lever with the proper screw, connected the rod from the pedal and BINGO, all sorted.

I hope by putting this on the forum it will save others from having to go through what I did. To cap it all the sun came out I could ride the damn thing again........RESULT.

[Mezzo Litro]

Having hellish time rebuilding same rear brake master cylinder on my 82 500. Replaced all the seals (thanks NLM), but still not returning. Does anyone have a diagram of what the internals should look like? (it's a Grimeca item), as suspect that the spring may be wrong or too weak.

Thanks

[snatchpiece]

I wish I had replaced the spring as well, I'm surprised it's not part of the seal/repair kit! I don't have a diagram, but even with the seal and the bore dripping with fluid it was still a tight fit and may need working up and down a bit to get it returning properly.

[Morizzi]

I'm only new to the forum and I relise that this thread is fairly old but I've also had issues with this.

I've come to similar conclusions about the spring but have overcome it with different strategies.

1. I modified a plastic cone to sit in the M/C and hold the spring as it is compressed. It also holds the spring off the end wall by a mm or two giving a bit more tension.

2. This is the one that really helped. DOT 3 fluid is glycol based. It is more slippery than DOT4 so the seals slide along the cylinder wall easier and don't bind.

3. I can gravity feed the fluid via the bleed nipple. I fit a hose to the nipple, fill it with clean fluid above the M/C level, crack the nipple and then the pressure switch on the M/C. Holding the level above the M/C the fluid will slowly fill and purge the air at the highest point of the pressure circuit.

4. I normally bleed by using pressure. I fill the reservoir then fit a cap with an air inlet installed. I use a normal domestic compressor but only a couple of psi. This allows me to crack the joins at will.

Cheers

Rod