Brake master cylinder - I don't know what I need to know...
Discussion
Evening ladies and gents. I'm after a bit of expert advice, or at least, someone to tell me if I'm missing anything incredibly important.
What I'm doing: I have a 1973 beetle chassis that's being converted into a... 'thing'. I need to change the standard tandem dual brake master cylinder for two individual master cylinders due to 'packaging issues'. It won't all fit as is.
What I don't know is, what I need to know to know if I'm doing it right.
My theory is:
The current tandem M/C is 19.05mm bore, twin 25mm throw. It has residual pressure valves fitted (obviously, because the car has drum brakes) and puts out equal pressure front and back, with different slave cylinders used to give the % of braking force split.
If I get 2 master cylinders @ 19.05mm bore and transfer the residual pressure valves in, and equally split the load from the pedal (bias bar set to zero) it should end up the same as having the tandem M/C right? And then I can further adjust bias bar to get a better split?
Am I missing anything important?
What I'm doing: I have a 1973 beetle chassis that's being converted into a... 'thing'. I need to change the standard tandem dual brake master cylinder for two individual master cylinders due to 'packaging issues'. It won't all fit as is.
What I don't know is, what I need to know to know if I'm doing it right.
My theory is:
The current tandem M/C is 19.05mm bore, twin 25mm throw. It has residual pressure valves fitted (obviously, because the car has drum brakes) and puts out equal pressure front and back, with different slave cylinders used to give the % of braking force split.
If I get 2 master cylinders @ 19.05mm bore and transfer the residual pressure valves in, and equally split the load from the pedal (bias bar set to zero) it should end up the same as having the tandem M/C right? And then I can further adjust bias bar to get a better split?
Am I missing anything important?
tapkaJohnD said:
Your bias bar is fighting a method of biassing the front/back split, different sized slaves, that it was not designed for.
Either:
make up a system that applies equal pressure to both masters. A rigid bar, not a balanced one
OR
use same size slaves front and back.
John
The bias bar will for the time being be locked once in the correct place to give as close to the original setup as i can hence mentioning the bias bar being set to zero. Another reason I'm doing it is to future proof it - it'll have disk brakes up front at some point.Either:
make up a system that applies equal pressure to both masters. A rigid bar, not a balanced one
OR
use same size slaves front and back.
John
Does the rest of the theory sound... sound?
The Wookie said:
If the balance bar is set central (presumably what the OP means by zero) then the pedal will apply equal force to the front and rear master cylinders
Exactly what I mean - zero bias, both clevis are equidistant from the pedal. 
thanka for the legal tip too - I'll be locking it in place one adjusted both now and when the disk conversion happens, just want it available when I do the conversion and as I said earlier, at the moment it's purely a packaging exercise!
My biggest concern was if it's okay to put the residual pressure valves over onto the new MC's as without those the wheel cylinders will leak.
GreenV8S said:
but it seems pretty unlikely that two completely different brake systems would have identical displacement/pressure characteristics.
My theory is that if you have 2 individual master cylinders with the same bore and throw as the original tandem master cylinder, and the same throw from the pedal (all of which is true on the parts I'm considering) the displacement/ pressure characteristics should be identical. Hydraulic theory that I've read supports this but I would like some confirmation that I'm not missing something!GreenV8S said:
I don't know where that theory comes from. The amount of pressure resulting from a given displacement will depend on the rest of the system i.e. the area of the slave cylinder, and the force/deflection characteristics of the brake mechanism, pads, drums/rotors etc. Those valves in the rear circuit would obviously have an effect too. It seems vanishingly unlikely that the front and rear circuits and brake assemblies would have identical characteristics given that they have completely different designs.
The rest of the system is exactly as it was - the only difference in the system at this point would be that is that instead of having a two master cylinder with 19.05mm bore, 25mm stroke in tandem, it would have two master cylinders with 19.05mm bore, 25mm stroke side by side. When it's later changed to disks, they are coming from a slightly later model and at manufacture the only change they made to the MC was to remove the residual pressure valves.The Wookie said:
Risk of dodgy side loads if you don't get it perfect and no guarantee of equal pressure as with different compliance characteristics for the same travel you could easily get a situation where one master cylinder is receiving more force than the other, hence more pressure
The actual manufacture of the pedal assembly is one thing I will have to be careful of, for sure. I'm using most of the original assembly, all I need to do to facilitate the change is shorten the bearing tube on the brake lever in the mill and reinsert the brass bush, the balance bar will then replace the clevisPeanut Gallery said:
Just my 2p worth!
The original system - 2 circuits, you mention one for the front, one for the rear - is it not one for Front Right and Back Left, the other for Front Left Back Right? - Yes, I fully understand that if they have been modified this may no longer be the case!
Is the "thing" going to be used on the road? - if so, please stop reading ... . . if not, do you need a dual circuit? - a single, larger diameter master cylinder might be small enough to fit into the gap, and you would not have to have balance bars etc.
I am not a brake expert in the slightest! (well, until I slammed on brakes when I only had my front left wheel doing all the braking - then I very quickly became an expert at swearing - Back right was greasy, the other circuit had popped a seal in the master cylinder)
No definitely front / back split as standard.The original system - 2 circuits, you mention one for the front, one for the rear - is it not one for Front Right and Back Left, the other for Front Left Back Right? - Yes, I fully understand that if they have been modified this may no longer be the case!
Is the "thing" going to be used on the road? - if so, please stop reading ... . . if not, do you need a dual circuit? - a single, larger diameter master cylinder might be small enough to fit into the gap, and you would not have to have balance bars etc.
I am not a brake expert in the slightest! (well, until I slammed on brakes when I only had my front left wheel doing all the braking - then I very quickly became an expert at swearing - Back right was greasy, the other circuit had popped a seal in the master cylinder)
Yes the thing is going to be used on the road lol. If it had been a few years earlier, it would have been single circuit from factory and that would both fit and work, but as you say, not something I'd want for the road!
I'm confused / obviously missing something - if I'm not changing the slaves at all why are they a factor? They are currently fed by two 19.05 x 25mm MC,s, and when changed they will still be fed by 19.05 x 25mm mcs just side by side instead of tandem. Sorry I may well be being totally thick just failing to see what you mean.
Ahhh okay now I see where you're coming from! I was told that the tandem system is equal displacement / pressure so couldn't work out what you meant, my bad! Accordikg to the info I have it's a single assembly front to back providing both circuits but I'll strip the original down and check tomorrow!
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