Rear master cylinder recomendations

[3-Max]

Yes me again with my '98 ******* trike.

I think I need a replacement rear master cylinder as what's on there atm is off ??? & weeping fluid (I know how it feels).

My Qs are:
Is the OEM unit considered good or prone to problems?
Would a new 'universal' be any better?
Any other aftermarket ones considered better?
Bearing in mind it'll be operating discs on two car hubs any thoughts? (sorry that is more trike territory but thought I'd ask anyway).

Ta.

 

[Fire-medic]

Not sure of the piston size, you may want to see what other people with VMax sidecars are running. You may need more piston diameter to have good feel for the size of the wheel cylinders on the two-wheel axle.

No the OEM is not known for giving trouble. I suspect most VMaxes go their entire life on the original rear master cyl. However, given your special needs, more knowledge is needed. What size is/are the piston(s) on the rear axle, and how many pistons are there?

 

[3-Max]

Thanks.
One piston per caliper. I think about 30mm. 9.5" discs. 15" rims.

 

[Fire-medic]

http://www.vintagebrake.com/mastercylinder.htm
Read this chart, use 30 mm for one piston, and you have two pistons/two calipers, I interpret this to be reading the 30 mm and two pistons cell, which is a value of 12.50. His description for a single piston is to find a value between 12:1 to 14:1. " Single piston calipers are much happier in the 14:1 to 12:1 range." The value associated with this is a 12 mm piston.

Anyone who has a difference of opinion on this, please provide your reasoning. FYI, I previously contacted the author of this chart, and he said that you should account for however many pistons you have, in however many calipers you have, being actuated by a single master cyl.

 

[3-Max]

Firstly : Fire-medic. A big thank you for the help you've been giving me. It really is very much appreciated.

And this chart has pointed me in the right direction & I'm in that rabbit hole now. It helps explain why the rear brakes feel so spongy. I've no idea what master cylinder is on there. O/D is 30mm.
I've no doubt at all it was just what they had in the parts bin. Unknown fluid condition & a weeping seal no doubt plays a part too.

I've also turned up this. I know it's about front brakes etc. but worth noting for others who read this thread (it's the same chart):

"Sounds like you're getting your information from the Vintage Brake chart page. For anyone reading along, here it is: Front Master Cylinder Ratio Chart
The thing is, Vintage brake is talking about racing motorcycles, two fingered power brakes with a light enough touch that there's no discernible input to the handlebars to throw you off line in a turn/curve. Street bikes are a different critter. Factory Harley brakes are adequate, but not great. Really good brakes might throw some inexperienced riders off, grab a handful and wash the front wheel out from under them. But there is room for improvement..."

I'll post more when I have it.

 

[3-Max]

So I have discovered (for cars):
That a disc brake system requires 900-1200 psi ( 0.63-0.84 kg per sq.mm) to operate efficiently. ie, this is the pressure in the hydraulic fluid.
For a road car the effort applied by the driver at the brake pedal is about 40lbs (18kg) of effort.
The RR brake pedal lever ratio for a (my) Vmax is 4.75:1

From there I've worked out what pressure I would get for each diameter master cylinder thusly (using metric as that's what the M/cylinders are measured in):

18(kg) x 4.75 = 85.5 kg of force applied to the master cylinder.
this divided by the piston area will give kg per sq.mm so for:

master cylinder size
11mm area = 95 sq.mm gives 85.5/95=0.9 kg/mm
12mm area = 126 gives 85.5/126=0.68 kg/mm
13mm area = 132 gives 85.5/132=0.65 kg/mm

If I drop the effort to operate the brake to 30lbs (13.6 kg) I get:

11mm ; 0.7 kg/mm
12mm ; 0.53 kg/mm
13mm ; 0.5 kg/mm

As I'm looking for 0.63 - 0.84 kg/mm it's looking like a 12mm M/cylinder would be the largest I should be looking at & maybe even an 11mm if I want easier effort on the pedal.
This looks to correlate well with the info from the chart.

Of note, my friend was just round is of the opinion that what's on there atm is from a car (it is a Brembo). It certainly looks quite large compared to pics I've seen of Vmax one.

Thinking about it I'm going ask on a trike forum to confirm all this. Probably what I should have done to start with but never the case with me.

 

[Fire-medic]

OK, thanks for doing the math, it appears the chart is accurate, and my use of it to recommend a 12 mm piston is also accurate.

 

[3-Max]

Well checking with trikers has just confused me. Over there they're talking about 3/4" (19mm) or 7/8" (20.5mm) as what should be used with this set up but I've only a couple of replies so far.
I think I'll get the exhaut & carbs fixed 1st then investigate what it is that's I've actually got, change the fluid (frnt & bak) see if that weeping can be fixed, then see how well they work.

 

[Fire-medic]

Those recommendations are so far off from what the chart says, I'd be suspicious. Maybe the large tire contact patch of the two wheels makes up for the loss of feeling or something. I wonder if those are the same guys running a Holley 850cfm double-pumper on their daily-driver SBC '63 Apache Fenderside pick-up?

 

[3-Max]

Yeah I am suspicious. The chart & every calc I've come across (2 of) are indicating 12mm or lower. Given the front end has 14mm M/C working 4 pots with an ave size of 32mm, a 19mm M/C for 2 pots at the back seems excessive. 19mm maybe good for a car but there are lot's of other things to consider such as shorter pedal, lower CoG & much much lighter.
If I run a calc for the trike as a 4 wheel car on the car tyre it says 11.5mm front & 9mm rr. If I do the same for a Vmax bike on bike tyres it says 9mm front & 10mm rr.
Overall I'm thinking the stock 1/2" Vmax or equiv is going to be pretty close if my guesstimate of 30mm piston is right. If they're 32mm pistons them it should be spot on. I ought to find out really.

What I've got is a Bendix - I (I for indigo) with the number 653 which I can't find any info on. Best I can tell, from appearance, it's a 19mm clutch M/C for a Fiat Uno.

 

[wayne billings]

Hi what ever happened to "dream v4", a very novel fellow?

 

[Fire-medic]

I'm sure you're familiar with "let sleeping dogs lie."

 

[3-Max]

Update for anyone following:
Well it looks like the trike guys win. I bought a s/h oem m/c & it did absolutely diddly squat. On full stroke wouldn't even start to operate the rr brakes. So I've made up some new bracketry for the 3/4" / 19mm clutch m/c (which I now believe is from a Fiat Uno), redone the pipework link up & externally fitted a fluid reservoir & bracket (there was nowhere else for it - oem size was inadequate for the larger m/c).
This works fine - so far.