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I presume the smaller volume inside the piston means it reacts more quickly to less 'squeeze' from the master cylinder.

Can't be that Ming,
The hollow section faces the pads, so fluid movement is identical. Possible that thicker walls distort the pads less? 'Feel' is always a difficult one to quantify from an 'engineering' standpoint.
George.

From an engineering point of view it's always the solution to a second order differential equation. Groans from engineers whoops of excitement from mathematicians.

The improvements in feel is down to the ratios! The smaller the master cylinder piston the greater the force transferred to the caliper pistons, for the same amount of lever pressure.
That's why fitting a second disc transforms the power and feel from that original master cylinder.
If you try and pump a larger volume of fluid it takes more effort than if you pump a small volume. Honest. There are better explanations by people with proper engineering qualifications but I can't even read them without getting a headache.

As for the replacement pistons in the last but one post's pic, the improvement is maybe down to the better fitting, non worn pistons and the fact you cleaned all the gunge out and replaced the seals?

My 79 Sport has twin discs, but still does not have much feel to the brakes, would a smaller cylinder be appropriate for twin discs?

pedro wrote:My 79 Sport has twin discs, but still does not have much feel to the brakes, would a smaller cylinder be appropriate for twin discs?

It would give more feel yes. You could possibly go a bit too far though! The original was meant for twin discs as on the 500 standard spec. (same master cyl.) I always though that set up was great, but I didn't want the extra weight on the 350. I suppose if you're more used to modern bikes all these will feel a bit dead.

If you want to feed the same system with a smaler piston the result is: A longer way of the lever (good for people like me with small hands, if you don´t like to brake with the finger tips) and less finger force to be needed.
I have had for a long time a 16 mm piston with dobledisc (no chrome left, and I prefer it like that) and stainless steel tubes. It works well but still a little wooden feeling. Then I changed a few years ago to a 13 mm grimeca pump with inclined reservoir for clipons (not easy to find) and since then I have a two finger brake, that might be even a little too "toxic" when braking some kind of frightend. That´s the reason for me suggesting a 14 mm piston. Perhaps it´s the right one, using only one disc.
I don´t realy have a hard point with the lever, but I´ve got used to it. The breaking is a lightyeas better, using only two fingers!

With the normal one disc models they used 16mm pistons. The models coming from the factory with dobledisc like the 500 came with 19mm pistons. The only reason I can see for this is, that someone thought that more volume is needed to move 4 instead of 2 pistons. But that realy is not necessary, at least if you use propper tubes

norbert

This is the pump I was talkimg about. I prepared a second one in case that I wouldn´t get used to the drum brake of Dulcinea. It´s quite a little one and therefore also very nice using fairings
25 pieces all screws included, missing the rubber cover for the switch

I have opened a second outlet "downstairs" to be able to use two direct tubes and a stoplight switch (what woud be dificult using the normal outlet provocing a conflict with the tacho)

George 350 wrote:Can't be that Ming,
The hollow section faces the pads.George.

D'oh! Of course. Sounded good at the time.

Well........I bit the bullet and bought one of Roy Thersbys super looking discs that he is selling on ebay and imediately got a horrendus amount of brake judder. It was so bad I ran the bike over to my local mot centre and put it on their brake testing machine and yes it would have failed the mot, we put a dial gauge on the disc and it was about 1mm out of true. I returned home and had 3 goes at taking the disc off and gently wet and drying the inside of the wheel in case there was any old paint or dirt behind the disc but it made no difference. I then refitted the original disc and the judder dissapeared but the braking was slightly better as I had fitted a pair of EBC sintered pads in the meantime. I rang Roy Thersby and he said the powder coating on the disc carrier may be to blame so I have sent the disc back to him and am hoping he will eventually send it back with the mounting plate machined. I also fitted a braided hose but the brake still feels pretty wooden, I will try a few more of your ideas and report back.

Vincent, sorry to hear of your disc problem. I'm sure Roy will get to the bottom of it.
A point about braided hoses though, they will make the brake feel more 'solid' if anything. The idea of the braid is to stop the hose liner expanding when you apply the brake. So the travel will get very slightly less. The old rubber covered hoses had more give in them and just got slacker as they got older.

Steve Brown wrote:A point about braided hoses though, they will make the brake feel more 'solid' if anything. The idea of the braid is to stop the hose liner expanding when you apply the brake. So the travel will get very slightly less. The old rubber covered hoses had more give in them and just got slacker as they got older.

The braid does nothing to limit expansion, it's just there to protect the pipe underneath, in fact you can slide it straight off if it doesn't also have the overmoulded plastic cover. The pipe core is made of a PTFE based material that expands less under pressure than the rubber hose with a fabric reinforcing layer.

Devil's advocate here: my Grimeca m/cylinder grew an annoying leak (scored bore?) and I really wanted to keep my bike as Italian as I could. I found a NOS Grimeca M/C that was 16mm instead of my old 19mm. Fantastic improvement. I then delighted in swapping the dodgy old rust-flinging Darmah discs I was using, having had my original twin Morini discs re-chromed at Philpots, while visiting the UK.

My braking has never been better - though I haven't tried it in a proper Auckland downpour yet.

So, a new M/C and new hard chrome equals new braking. It's not fair to go comparing 40-year-old components with modern fare and blame the design. I also happily swapped my cheapo old Hagons back for rebuilt Marzocchis.

Italian all the way for me.

norbert wrote:You better look for a smaller like a 14 mm pump, then the rest can stay as it is and stainless tubes

norbert

Maybe not in the UK but in other places it is well known that fitting the 250 master cylinder to the 350 is a big improvement. And uses all Morini parts.

Gary&Anne

Habilis wrote:The Original master cylinder was awkward to fill with clip ons , i had to lean the bike over to get it full enough to bleed.

If anyone is having this issue, it's best to start off by putting the bike on the sidestand and steering all the way right. This more or less levels up the reservoir. Depending on how you have it positioned on the clipons. Now it's level you can take off the lid and fill, check, bleed, etc in a way that just isn't possible on the mainstand.

Gary and Anne