Trapezoidal Frt Master Cylinder - Removal of Cover

[Cdn850T5NT]

Hey, in reference to Lemans IV's and T5's - they have, as stated above, trapezoidal cover master cylinders.  The flathead Allen head bolts that hold the cover down can be problematic, if i) the o-ring at the base of the plastic reservoir starts leaking, or ii) if the bolts are not put-in with a smidgeon of grease when putting the cover back on.  Why?  'cause brake fluid, being hygroscopic, and when in contact with the threadings-in to the aluminum reservoir holder casting, will cause the bolts to rust-up... OR rainwater that gets in (easily enough) also causes rust.

The solution, which is a PITA, is to drain as much brake fluid as possible thru the caliper bleed screw (which of course, makes subsequent bleeding a pain), disconnect the hydraulics hose from the banjo fitting, and to remove the master cylinder without removal of the cap.  It goes without saying that before you do this, liberally protect your paintwork first!  Then turn the assembly upside down on your bench, and dose all thread penetrations (i.e. all four - which penetrate thru) with a good quality penetrating oil over several days.  Then attempt to key the heads and carefully unthread them.  This SHOULD work.

Those Allen heads are small small small - and easily strip.

I don't know for sure, but I would bet it'd be hard to get replacement screws... so attempting to salvage your original ones - would be best.

[Kiwi_Roy]

That looks nasty, would you be better off with stainless or brass bolts?

[larrys]

Yes to stainless fasteners. And use grease instead of anti-seize. You already have two dissimilar metals, the metals in anti-seize would add more to the mix.
Larry

[Cdn850T5NT]

That looks nasty, would you be better off with stainless or brass bolts?

Not my master cylinder... but the pictures from the 'Net.

I agree, stainless would be best, and assemble with a bit of grease.

Thx for the responses...

Cdn

[Cdn850T5NT]

That whole master cylinder / front brake lever / switches combination is a bit of a problem... in that new master cylinders... and even kits are NLA.  Early on in the life of my bike - I got a replacement cylinder to have at the ready.  I suspect, though, that as the size of the Master is PS-12 - one could find a suitable piston, seals kit... if the bore (if necessary) were sleeved to brass.

The switches are worth preserving, using relays, so that a whole lot of sparking does not occur within the switches themselves.  Chuck in Indiana did the definitive rebuild of those switches... so they ARE rebuildable... but I believe, worth preserving.

All of this - if a person is interested in originality.  If not, all manner of alternatives are available.

[guzziart]

I rebuilt mine in 2011.  Moto International sold me the piston rebuild kit with cover gasket, oring, cap, etc.  I would have bought double but the parts fellow mentioned that I probably got the last one in existence.  I'm not looking forward to having to replace those parts or modify all that in the future.  I've been diligent in performing fluid changes over the years, don't know what else I could do to prolong service.  Too bad MG or Brembo don't offer (new & improved)replacement parts or the whole assembly.

When I did the master cylinder I also serviced the switches and then added relays to eliminate higher current from going through them.  Although, it seems to me that someone was offering knock-offs of the original switches recently.

Anyway, if I ever do have to replace the the Trap Master, I'll be back here to solicit advice from y'all.

[Cdn850T5NT]

Guzziart, thank you for recounting your experiences.  I have to assume when you refreshed yours, you found the bore OK.  I expect that the aluminum bore is anodized?

Oh, good on you, to have gotten a spare cap and presumably a top-gasket; I wish I had at least the top-gasket as a spare.

Going forward, though the whole assembly is pretty darned small by way of bore and length, it seems to me that it MAY be possible to take one that is found to be in poor condition, and bore it and sleeve it to brass...  Then, I suppose with some tomfoolery a currently-available PS-12 piston could be fit.  The cross section, attached, shows the detail of where the main feed-port is, versus where the bypass port is...

If memory serves, an outfit called Whitepost Restorations (who specialize in restoration of old, old cars, which often have four wheel drum brakes) - they sleeve NLA wheel cylinders to a brass bore.  That is not to say that they could do something as small as a trapezoid Guzzi master... but it might be worth asking.

https://whitepost.com/brake-sleeving-rebuilding-services/

Cheers!


posted image

[guzziart]

As I recall, the bore was OK.  The reason for disassembly was that the bike hadn't been ridden by the PO in over a decade and it was obvious something was wrong with the trap master because everything was wet with brake fluid.  I used all of the new parts and believe I kept all of the take off bits but would be reluctant to attempt to reuse because those items were already 20+ years old & wouldn't seal.  And, the mc cap/cover was distorted from age... I suspect.

I like your idea of procuring a take off assembly and attempt to get it re-sleeved, I may act on buying one should one appear on ebay, etc.  However, replacing/obtaining the sealing boot between the cap & reservoir would still be a problem.

I know of a member on the V11 board that has a problem with his LMIV trap master and is in a quandary as to how to resolve the problem.

[Cdn850T5NT]

Seems to me if the collapsable/bellows top seal is NLA then you could improvise. Take aluminum plate, moderate thickness, and duplicate the hole pattern of the original cap. Drill and tap the middle 1/8" NPT. Later fit a flexible tube vent with a "weatherhead"  to prevent ingress of moisture... and clip same to the nearby windscreen. Then for a gasket use EPDM sheet, appropriate thickness and durometer hardness, and punch a hole in the middle of it (to coincide with the tapped hole). Use the appropriate brake fluid compatible caulk on its top surface only. This whole setup will work satisfactorily but because it is vented to atmosphere it will demand yearly or 6 mo. frequency brake fluid changeouts.

The other comment is how many brake master cylinder reservoirs are truly sealed from atmospheric moisture?  I had a domestic, fairly late model car, that had a cap with a bellows seal.  Well, this bellows seal had a hole deliberately punctured in its top... where it truly DID communicate with the outside air.  This was not a fluke... all of these cars did, in the same manner.  So what I am saying - is that it is likely that even 2 year brake fluid changeout intervals, with the above-indicated set-up, probably would be fine.

[Cdn850T5NT]

Here is the seal kit (albeit minus the actual piston) for the trapezoidal frt master: https://www.motogiovane.com/mobile_product_info.php?cPath=67&products_id=772

It's from "motogiovane" in Italy. See attached screenprint. Note that a person strictly would have to turn on a lathe the cone shaped seal mounting "fixtures" to get these seal elements onto your old piston or your newly mfr'd piston without damage to the seals (and with liberal use of "brake grease").